共沸物，又称恒沸物，是指两组分或多组分的液体混合物，在恒定压力下沸腾时，其组分与沸点均保持不变。这实际是表明，此时沸腾产生的蒸汽与液体本身有着完全相同的组成。共沸物是不可能通过常规的蒸馏或分馏手段加以分离的。并非所有的二元液体混合物都可形成恒沸物。这类混合物的温度-组分相图有着显著的特征，即，其气相线(气液混合物和气态的交界)与液相线(液态和气液混合物的交界)有着共同的最高点或最低点。如此点为最高点，则称为正恒沸物；如此点为最低点，则称为负恒沸物。大多数恒沸物都是负恒沸物，即有最低沸点。

常见的共沸物   

共沸物                 组分的沸点（度）                  组成(w/w)              共沸点（度）    

水－－乙醇              100－－78.5                     5－－95                    78.15   
水－－正丙醇                －－97.2                     28.8－－71.2             87.7   
水－－异丙醇                －－82.4                     12.1－－87.9             80.4   
水－－正丁醇                －－117.7                   37.5－－62.5             92.2    
水－－异丁醇                －－108.4                   30.2－－69.8             89.9    
水－－叔丁醇                －－82.5                     11.8－－88.2             79.9   
水－－异戊醇                －－131.0                    49.6－－50.4            95.1    
水－－正戊醇                －－138.3                    44.7－－55.3            95.4    
水－－氯乙醇                －－129.0                    59.0－－41.0            97.8   
水－－乙醚                   －－35                        1.0－－99.0              34   
水－－乙腈                   －－81.5                     14.2－－85.8             76   
水－－丙烯腈                －－78.0                     13.0－－87                70.0   
水－－甲酸                   －－101                      26－－74                  107   
水－－丙酸                   －－141.4                   82.2－－17.8             99.1            
水－－乙酸乙酯             －－78                        9.0－－91                 70   
水－－二氧六环             －－101.3                   18－－82                   87.8   
水－－氯仿                   －－ 61.2                    2.5－－97.5               56.1   
水－－四氯化碳             －－77.0                     4.0－－96                  66.0   
水－－二氯乙烷             －－83.7                     19.5－－80.5             72.0   
水－－苯                      －－80.4                     8.8－－91.2               69.2    
水－－甲苯                   －－110.5                   20－－80                   85.0   
水－－二甲苯                －－137－140.5           37.5－－62.5             92.0    
水－－吡啶                   －－115.5                    42－－58                 94.0   
水－－二硫化碳             －－46                         2.0－－98.0             44    

甲醇－－二氯甲烷     64.7－－41                        7.3－－92.7             37.8   
甲醇－－氯仿                 －－56.2                     12－－88                 55.5   
甲醇－－四氯化碳           －－77.0                     21－－79                 55.7   
甲醇－－丙酮                 －－56.2                     12－－88                 55.5   
甲醇－－苯                    －－80.6                     39.1－－60.9            57.6   
甲醇/甲酸甲酯/环己烷                                       17.8/48.6/33.6         50.8   
                    
乙醇－－乙酸乙酯     78.3－－78.0                     30－－70                 72.0   
乙醇－－苯                     －－80.6                     32－－68                68.2   
乙醇－－氯仿                 －－61.2                      7－－93                  59.4   
乙醇－－四氯化碳           －－77.0                      16－－84                65.1   
乙醇/苯/水             78.3/80.6/100                     19/74/7                  64.9   

乙酸乙酯－－四氯化碳 78.0－－77.0                    43－－57                75.0   
乙酸乙酯－－环己烷                                          46－－54                71.6    
乙酸甲酯－－环己烷                                           83－－17               54.9   

氯仿－－丙酮        61.2－－56.4                         80－－20                64.7   

甲苯－－乙酸        101.5－－118.5                      72－－28                105.4    

其他如： 
HFC365fc、二氯甲烷、甲醇 
HCFC141b、甲醇或乙醇 
nPB、异丙醇 
等等