The amount of oil washed from the partially retorted and unretorted shales from 14 interrupted runs of the controlled-state retort has been used to estimate dynamic oil film thickness on those shales. The data obtained indicated that factors that affect oil viscosity determine oil film thickness. For example, in the heated region of the retort, temperature was the controlling factor for oil film thickness. In the unheated region controlling factors included proximity to the heated region, gas composition and flow rate, retorting advance rate, and breadth of retorting zone. Factors that affected oil composition and thereby increased viscosity, such as increased gas velocity, oxygen in the retorting gas, slower retorting advance rate, and thinner retorting zone, increased oil film thickness. In the unheated region of the retort the oil film was thickest nearest the heated region gradually tapering to a more or less constant value approximately 1 meter from the heated region. Oil shale particle size did not affect oil film thickness.