This paper presents the results of an experimental program in which a Texaco L-163S engine was fueled with methanol and operated in its traditional stratified charge mode and then modified to run as a homogeneous charge spark ignited engine. The primary data taken were the aldehyde and unburned fuel emissions (UBF). These data were taken using a continuous time-averaging sampling probe at the exhaust tank and at the exhaust port and with a rotary time-resolving sampling valve located at the exhaust port. The data are for two loads, 138.1 kPa (20 psi) and 207.1 kPa (30 psi) BMEP and three speeds, 1000, 1400 and 1800 rpm.The data indicate that for both the stratified charge and the homogeneous charge modes of operation formaldehyde was the only aldehyde detected in the exhaust and it primarily originated in the cylinder. Over the load and speed ranges tested, the unburned fuel emissions were higher and the formaldehyde emissions lower when comparing the stratified charge to the homogeneous charge operation. Time resolved data at the exhaust port for the two modes of operation are compared. The homogeneous charge operation had a uniform concentration of both unburned fuel and formaldehyde over the exhaust stroke. The stratified charge data shows large changes in the unburned fuel and moderate changes in the aldehyde concentrations during the exhaust stroke.