The surface structure of propellants containing various oxidizers and polyurethane binder burning at pressures between one atmosphere and 800 psia is presented. These propellants, which contained ammonium perchlorate, potassium perchlorate or cyclotetramethylenetetranitramine as the oxidizer, were viewed while burning using cinephotomicrography, and as quenched samples using a scanning electron microscope (SEM). From these data the microstructure of the burning propellants was inferred. Results not only show why certain phenomena, such as the self-quenching of ammonium perchlorate-polyurethane propellants, occur, but also serve as a basis for checking existing analytical models and developing more accurate models for solid propellant combustion.