The effects of a H2PtCl6 additive on the rate of formation of coke deposits on quartz and Incoloy surfaces were investigated in the steam cracking of ethane and propane in a continuous-flow thermogravimetric analyzer (TGA). The TGA operating conditions were 1 atm pressure, 820-845 degrees C temperature range, about 1.5 s reaction time, and steam to hydrocarbon molar ratio of about 2. Specific coke formation rates consistently decreased in the presence of the additive for both ethane and propane pyrolysis. For example, in ethane pyrolysis the specific coke formation rate on the quartz surface at 830 degrees C decreased from 0.34 mu g/cm(2).min in the absence of the additive to 0.089 mu g/cm(2).min in the presence of the additive, representing an improvement by a factor of about 4 in coking rates. On the Incoloy surface, coke formation decreased from 0.98 to 0.38 mu g/cm(2).min. For the case of propane pyrolysis at 830 degrees C, the coke formation rate decreased from 0.51 to 0.33 mu g/cm(2).min on the quartz surface and from 1.9 to 1.0 mu g/cm(2).min on the Incoloy surface.