A theoretical and experimental investigation of the dynamic mechanical properties of polymeric films in liquid media has been conducted. In an analytical development the effect of a viscous medium surrounding the specimen on the dynamic mechanical measurement is determined by calculating the viscous drag on the specimen which is subjected to a sinusoidal oscillation; this result is incorporated with the dynamic modulus analysis. The resulting expressions are demonstrated to predict the effective dynamic mechanical properties of films in liquid media by knowing the dimensions, viscosity, and density of the liquid and the frequency of oscillation. In order to separate the viscous drag from any polymer‐solvent interactions, a series of experimental studies using the Vibron instrument was conducted to measure the effective dynamic mechanical properties of films in a variety of noninteracting liquid media and dry conditions. The resultant dry measurements were used along with the theory to show the effect of viscous drag on the dynamic mechanical properties of a film in a liquid medium.