Simulation study has shown oil production is accelerated when propane is used as an additive during steam injection. To better understand this phenomenon, distillation experiments were performed using San Ardo crude oil (12oAPI). For comparison purposes, three distillation processes were investigated: dry-, steam-, and steam-propanedistillation, the latter at the propane-to-steam mass ratio of 0.05 at steam injection rate 0.5 g/min. Two sets of the distillation experiments were carried out. In the first set of experiments, the distillation temperatures ranged from 115ºC to 300ºC. Distillation pressures ranged from 0 psig to 998 psig for steam- and steam-propane distillation. The temperature-pressure combination used represented 15ºC superheated steam conditions. In the second set of experiments, the distillation temperatures ranged from 220oC to 300oC at 260 psig. The temperature pressure combination used represented field conditions for crude oil. For both conditions, the cell was kept at each temperature plateau (cut) until no increase occurs in distillation yields. Distillation yields were collected at each cut, and the volume and weight of water and hydrocarbon measured. Based on these experiments, a thermodynamic modeling framework was developed that describes distillation effect and oil production for steam distillation experiments. The model is based on composition of crude oil, molecular weight of heavy fraction. The analytical model results are compared against the experimental data for synthetic crude and crude oil to verify the validity of the model. Main results of the study may be summarized as follows. The yields for steam distillation for saturated conditions of Tsat+15 o C and Psat is 10 % and with addition of 5% of propane to steam no significant increase occurs in distillation yields. The yields for steam distillation for field conditions of 260 psig and temperature range (220 ~300oC) is 18 % and with addition of 5% of propane to steam no significant increase in distillation yields. The results indicate that propane has minimal distillation effect on the heavy oil. This occurs possibly because of lesser amount of light fractions in the heavy oil that enhance the separation of components in the oil caused by the concentration gradient.