Abstract : This document reports on the preliminary investigation of relative humidity effects on light absorption by aerosols. The Desert Research Institute took part in the Southern Great Plans Radiation Measurement and took simultaneous measurements of photoacoustic light absorption and light absorption measured by filter methods as a function of relative humidity. The variation In relative humidity was controlled artificially. This work is described in detail in Section I. A detailed theory of the effect or evaporation condensation on photoacoustic output has been developed at the University of Mississippi This theory is valid over a wide span of particle sizes, particle densities and operating frequencies. This work is presented in Section II.