An experimental study of size distribution effects on burn time measurements using optical diagnostics was conducted in the UIUC heterogeneous shock tube with aluminum powder distributions between 3.1 and 11.2 μm in water vapor. Mixtures were created in order to test distributions between these limits. Mass averaging (instead of number averaging) of particle diameter is necessary in order to observe increasing burn times with diameter. Additionally, the number of particles observed in the collection volume of optics precludes number averaging based on probability of observation. In water vapor, burn times increase with pressure, and there is no indication that this observation is caused by a size distribution effect. Finally, low diameter dependences on burn time cannot be attributed to broad overlapping size distributions.