The results of an experimental investigation of flame spread over thermally thick composite samples are presented. The aim of the study is to establish the influences of material property variation and finite rate chemistry on the rate of spread and also to obtain the dependence of the chemistry factor on the oxygen mass fraction in the oxidizer flow. To this end, flame spread rates over samples inclined at various angles from the vertical direction are first measured in an oxygen environment and the results used to establish the "sole" influence of streamwise gravity on the spread process. A second set of flame spread tests are then performed using composites of various compositions within an oxidizer flow diluted with nitrogen. Finally, the results of both sets of experiments are incorporated into an existing flame spread rate formula to deduce the influences of material property variation and finite rate chemistry on the spread rate. The results show a good qualitative agreement with previous studies and produce new insights into the nature of the chemistry factor that can aid further development of the underlying theory.