The reciprocal of the reverberation timeTis expressed as a power series in the absorptivity of the enclosure. The leading term of the series is found to be the Sabine–Franklin expression, and the procedure is thus interpreted, for weakly absorbing enclosures, as a new derivation and confirmation of that still controversial expression. The next term of the series then provides a correction to the Sabine–Franklin value in many practical cases of intermediate absorptivity where the Sabine–Franklin value is reasonably accurate but not accurate enough. Four other formulas which have been advanced as improvements on Sabine–Franklin are shown to yield unreliable corrections which are even of the wrong sign in some enclosures. Finally, it is shown thatTbears a simple relation to the gaingof a laser, and, thus, existing evaluations of eithergorTare immediately interpreted as evaluations of the other.