Studies of the change in mass and the geometry, due to evaporation, of small droplets of water on poly-(methyl methacrylate) (PMMA) have been performed. Measurements of contact angle, θ, contact radius, r0, and rate of mass loss with time in the regime of constant contact radius, valid for θ< 90°, are reported. Contrary to previous reports in the literature, it is shown that the rate of mass loss is proportional to the height of the droplet and not the spherical radius. The results are explained by a model based on a spherical cap geometry and the observed constant value of the contact radius. The predicted variation in contact angle with time is accurately fitted by the derived formula which gives a slope proportional to r0-2. Experiments over a range of contact radii from 0.293 to 0.585 mm confirm this power law behavior. To first order the rate of evaporation is predicted to have a weak time dependence and to be proportional to r0; a result which is also supported by the experimental data.