The theory of the dependence, on the molecular orientation, of the velocity of photochemical reactions in monolayers at the surface of aqueous substrates is developed. The molecular extinction coefficient in the film is discussed and a definition is given of the ``apparent quantum efficiency.'' Consideration of the molecular thermal agitation suggests the possibility of a phase boundary potential gradient of the order of 108 volts per cm. Under certain conditions, the apparent quantum efficiency is proportional to the absolute temperature. Factors modifying the absorption spectrum of a compound in a monolayer are discussed. By an application of the general theory of radiation, a relation is obtained between the molecular orientation and an integral of the molecular extinction coefficient in the film over the absorption band. The molecular extinction coefficient in the film cannot be deduced a priori from that in solution; it is proportional to the mean value of sin2θ (where θ is the angle between the molecular dipole axis and the direction of the incident light) and is usually of the order of 3/2 sin2θ that in solution. The theoretical accuracy of the sin2θ relation is discussed and both quadrupole absorption and reabsorption of coherently scattered radiation are shown to be of no practical importance. Direct measurement of the absorption spectrum of a monolayer is essential for the absolute determination of the quantum efficiency of a photochemical reaction.