General formulas for the absorption probability from the process $\ensuremath{\gamma}+{\ensuremath{\gamma}}^{\ensuremath{'}}\ensuremath{\rightarrow}{e}^{+}+{e}^{\ensuremath{-}}$ are given for high-energy photons traversing isotropic photon gases having various spectra. Asymptotic formulas are derived and general results are given in graphical form for photon spectra having blackbody and power-law form. The results are applied in a following paper to calculate the absorption probability for high-energy photons traversing cosmic distances.