Monthly precipitation data for about 150 stations in Mexico were used to quantify spatio-temporal differences in the North American Monsoon region. Geostatistical techniques were applied to three indices of monsoon precipitation, namely the standardized difference between April and May, May and June, June and July precipitation. Co-kriging between those indices and the percentage of annual precipitation falling during the summer monsoon was used to map the spatial separation and progression of the monsoon. This objective classification of monsoon-affected land areas can easily be assimilated into GIS (Geographic Information Systems) databases, and provides a useful baseline for the evaluation of past and future changes in water cycle variability within the North American Monsoon region.