In this letter we report the study of surface chemical reactions with scanning force microscopy (SFM) with chemical specificity. Using chemically modified SFM probes, we can determine the local surface reaction conversion during a chemical surface modification. The adhesion forces between a functionalized tip and a chemically modified surface were used to calculate local reaction conversions on a molecular level on model substrates. Moreover, the distribution of reacted (and unreacted) functional groups could be mapped by chemically specific lateral force imaging. This technique was applied to study surface modifications of polyethylene films upon etching with chromic acid.