Monte Carlo simulation techniques have been carried out in the isothermal-isobaric ensemble to study the thermodynamics of mixing and the extent of self-association occurring in methanethiol/propane and ethanethiol/propane mixtures. The OPLS potential energy parameters developed by Jorgensen were used to model the thiol-thiol, thiol-propane, and propane-propane interactions. The mole fractions of the mercaptan in the mercaptan/propane mixtures were 0.0, 0.0625, 0.125, 0.25, 0.5 and 1.0. The density, excess enthalpy and excess volume at 231 K and 1 atm are reported for the different concentrations, and the results show that non-ideal mixing behavior exists in these systems. Analysis of hydrogen bonding between the mercaptan molecules shows that substantial hydrogen bonding exists in all the solutions studied, the degree of which increases with increasing mercaptan concentration. At the lowest concentration for methanethiol and ethanethiol, there is an average of 0.6 hydrogen bonds per thiol molecule, while for pure methanethiol and ethanethiol the average number of hydrogen bonds increases to 1.0. The fraction of mercaptan molecules which exist as monomers decreases as the mercaptan concentration increases.