The purpose of this study was to investigate hydrocarbon emissions from the in-cylinder post-injection effect on NO oxidation over a diesel oxidation catalyst (DOC) with various feed mixtures. In the temperature ramp experiments with varying C3H6 concentrations in NO/O-2 and NO/O-2/H-2 mixtures, addition of C3H6 was observed to inhibit NO oxidation by delaying the ignition temperature of NO. Furthermore, NO2 production was decreased with an increase in the feed C3H6 concentration. NO was able to reduce platinum oxide and produce NO2 at 200 degrees C. Experimental results with C3H6 switching in/out at 350 degrees C indicated that the introduction of 1.1% C3H6 clearly increased the catalyst temperature and decreased the NO2 field. Feeding in H-2 led to a higher NO2 field and a lower NO2 field drop. The decline in the NO2 concentration can be attributed to platinum passivation caused by H-2 in the NO/O-2/H-2 mixture.