The slow growth characteristics of a condensing ethanol sessile drop on a quartz substrate were studied experimentally. Using interference microscopy measurements of the transient liquid film profile (curvature) to obtain the pressure field and a Kelvin-Clapeyron model of interfacial mass flux to obtain the interfacial temperature difference, changes in the apparent contact angle were related to interfacial subcooling and, therefore, adsorption. We found that while the radius of curvature of the growing drop increased linearly with time, the apparent contact angle remained the same during slow growth at a constant condensation heat flux. The radii of curvature and the apparent contact angles at different axial locations were measured and compared. The results demonstrated that the curvature, the contact angle, the interfacial subcooling, the interfacial mass flux, the spreading velocity, and adsorption are coupled at the moving contact line. Motion and the apparent contact angle are governed by the condensed adsorbed thin film thickness and curvature at the contact line.