—The unsteady one-dimensional cyclic absorption and desorption of a dilute amount of gas in a thin (constant-thickness),isothermal (constant-property) liquid layer on an impervious noncatalytic wall is examined. The pertinent conditions are taken to be (1) thermodynamic equilibrium (such that Henry's law holds at the two-phase interface), and (2) diffusional resistance of the liquid layer being rate-controlling (such that the problems for the liquid layer, and for the bulk gas contiguous to it, decouple). Specifically, the mass fraction of the gaseous species of interest is taken to be known, and of square-wave periodic character, at the two-phase interface, so attention is concentrated on the total gaseous content of the liquid layer as a function of time. While start-up (in which the liquid layer is taken to be initially free of gaseous content) is considered, primary interest is in steady periodic operation in which no residual influence of the initial condition remains. The model is developed as one step in a series of simplistic characterizations of hydrocarbon absorption/desorption in the motor oil coating side walls in reciprocating-piston-type internal-combustion-engine cylinders, of concern as a possible prime source of undesirable unburned-hydrocarbon emissions in the cylinder exhaust. The role of spark timing and of engine speed on the phenomena is given focus, and, in particular, the experimentally observed decrease of unburned-hydrocarbon content with increasing rpm is consistent with the model.