Pesticide sorption in soil involves many factors, such as pH, soil redox, the amount and type of humic acids (organic matter), clays and metallic species. Little work in carbonatic soils has been done to study the effect of the above mentioned factors on the sorption of pesticides. Studies were conducted to determine the interaction between these factors and the carbonatic soils. Twelve carbonatic soil series and nine non-carbonatic soil series showed that redox properties for carbonatic soils tended to be smaller and, contrary to the non-carbonatic soils, many of them with negative potentials. We observed a positive correlation between water extractable iron versus diluted acid extractable iron for the carbonatic soils (R2 = 0.88) but not for the non-carbonatic soils. This finding suggests that the reductive environment of carbonatic soils favors the ferrous species (Fe+2) rather than the ferric (Fe+3). These factors apparently affect the solubility of the organic matter, thus decreasing the sorption of pesticides in carbonatic soils compared to that of non-carbonatic. We observed a positive correlation between the extinction coefficients (k) at 280 nm for the humic acids of carbonatic soils and the sorption coefficients of atrazine and diuron (R2 = 0.61 and 0.56, respectively). The k values at 300 nm did not show a significant correlation. The averaged pKa values for humic acids from the soils in this study were similar. However, the first derivative titration plots of humic acids showed that humic acids from soils in this study contained several kinds of functional groups with different pKa values, and also suggest that the properties of the functional groups are different. The data suggest that the aromatic rather than the aliphatic content of humic acids seems to have a predominant role in the soil sorption of pesticides. These findings indicate the importance of studying more in depth the sorption, mobility and degradation of pesticides in carbonatic soils in order to protect the water resources.