The limiting conditions consistent with p-process synthesis in supernova envelopes are inferred from calculations of the appropriate rates of proton capture and neutron photodisintegration. Temperatures in excess of 2 x 10 to the 9th power K are required, for proton mass densities of the order of 100 g cm to the minus 3rd power, if significant production of p-process nuclei is to take place on a hydrodynamic time scale. It is concluded that these processes must be appropriate to very different stellar or supernova environments.