Cryogen spray cooling (CSC) is used to minimize the fisk of epidermal damage during dermatologic therapy. However, the dominant mechanisms of heat transfer during the transient cooling process are incompletely understood. The objective of this study is to elucidate the physics of CSC by measuring the effect of initial substrate temperature (T-0) on cooling dynamics. Cryogen was delivered by a straight-tube nozzle onto a skin phantom. A fast-response thermocouple was used to record the phantom temperature changes before. during and after the cryogen spray. Surface heat fluxes (q") and heat-transfer coefficients (h) were computed using an inverse heat conduction algorithm. The maximum surface heat flux (q"(max)) was observed to increase with T-0. The surface temperature corresponding to q" also increased with T-0 but the latter has no significant effect on h. It is concluded that heat transfer between the cryogen spray and skin phantom remains in the nucleate boiling region even if T-0 is 80 degreesC.