A methodology to infer skin soil moisture from time series of passive microwave imagery is presented. Based on temporal changes in the microwave polarization difference temperature, it yields a quantitative moisture estimate as a fraction of field capacity. Based on a processing technique that separates low-frequency components caused by vegetation changes from fast precipitation-induced changes, its main feature is the complete independence of ground-based data. The method solely employs information contained in the microwave signal itself. When applied on Special Sensor Microwave/Imager imagery over the European continent, it is found that the methodology is only applicable to areas with sparse vegetation cover, where the fractional error (with respect to field capacity) is estimated to range from 16 to 28%. Finally, comparison of calculated moisture time series with observed precipitation shows a fair agreement.