Methanol, a popular alternative fuel candidate, can theoretically be dissociated on-board a vehicle into a 2/1 molar mixture of hydrogen (H2) and carbon monoxide (CO) having a 14 percent greater heating value than that of methanol vapor. In this study, engine efficiency and fuel consumption with methanol vapor and dissociated methanol (simulated by a 2/1 mixture of Ha and CO) were compared in a single-cylinder engine at equivalence ratios (Φ’s) ranging from 0.5 to 0.9 and compression ratios (CR’s) from 11 to 14. Whan compared at the same Φ and CR, the reduction in fuel consumption for dissociated methanol compared to methanol (3-7 percent) was smaller than would be expected based on heating value alone. Indicated thermal efficiency with dissociated methanol was only 0.89-0.55 times that with methanol.Thermodynamic analyses were conducted to isolate the factors responsible for lower efficiency with dissociated methanol. The largest factor was dissociated methanol’s lower molecular weight (10.68 compared to 32.04 for methanol), causing an increase in compression work. Also, the higher heating value of dissociated methanol caused an increase in heat losses from the engine. Factors opposing the loss in efficiency with dissociated methanol were its faster burning rate and higher heat capacity per unit mass.