  We perform analytic linear stability analyses of an interface separating two
stratified media threaded by a radiation flux, a configuration relevant in
several astrophysical contexts. We develop a general framework for analyzing
such systems, and obtain exact stability conditions in several limiting cases.
In the optically thin, isothermal regime, where the discontinuity is chemical
in nature (e.g.\ at the boundary of a radiation pressure-driven H \textsc{ii}
region), radiation acts as part of an effective gravitational field, and
instability arises if the effective gravity per unit volume toward the
interface overcomes that away from it. In the optically thick "adiabatic"
regime where the total (gas plus radiation) specific entropy of a Lagrangian
fluid element is conserved,for example at the edge of radiation pressure-driven
bubble around a young massive star, we show that radiation acts like a modified
equation of state, and we derive a generalized version of the classical
Rayleigh-Taylor stability condition.
