An experimental study of flame-spreading behavior over thin Mg/PTFE/Mg (magnesium/poly-tetrafluoroethylene/magnesium) films with a thickness of 10/25/10 μm was conducted in both a pure argon and a 10% oxygen/90% argon environment. The objectives of this research was to determine the effect of initial chamber pressure, gap width between two adjacent films, and ambient gas composition on the flame-spreading rate (VFS) of Mg/PTFE/Mg pyrotechnic films. The flame-spreading rate was deduced from the flame-front trajectory measured by an array of fast-response lead-selenide (Pb-Se) infrared photodetectors. The initial chamber pressure was varied from 0.1 to 20.8 MPa. For tests with two films aligned in parallel, the gap width was varied from 50 to 400 um. For single-film tests, VFS was found to decrease as the initial chamber pressure increased; this observed trend is believed to be caused by a greater heat loss to ambient gases in a higher pressure environment. For double-film tests, results showed the existence of an optimal gap width for which VFS is maximized at a given initial chamber pressure and an optimal initial chamber pressure for which VFS is maximized at a given gap width. These results are caused by a combination of several factors, including the required physical space for gas-phase reactions and the heat transfer rates to both unburned films and ambient gases. Addition of oxygen in test environments was found to decrease the flame-spreading rates when compared to those in pure argon. Experimental correlations relating the flame-spreading rate to the initial chamber pressure were developed for both single- and double-film tests.Keywords: Magnesium; polytetrafluoroethylene; flame-spreading rate; photodetector