The present research, considered exploratory by the Author, was dons in order to study leaf invertases of two coffee varieties, Geisha (SH1SH5) and Kent SH2SH5) inoculated with races III (v1v5) and XXIV (v2v4) of Hemileia vastatrix. The Geisha coffee is susceptible to race III and resistant to race XXIV, while Kent coffee has opposite reactions. In each pair of leaves, one was inoculated and the other was kept as control. Determinations of the invertases were made at 6 and 16 days after inoculation. The preliminary results showed that in the susceptible combinations there was no changes in the invertases, that were the same as in the healthy plants, with the maximum of activity at pH 4.5. In the resistant combinations, however, it was found an "acid invertase pH 4.0" on the 6th day after inoculations. This invertase was absent in both healthy and infected plants of the same varieties. The occurrence on the "acid invertase pH 4.0" is related to the decrease in the sucrose content of the leaves. The Author suggest three hipothesis that could explain a relation of this "acid invertase pH 4.0" with the coffee rust pathogenesis. These hipothesis, which could occur alone or simultaneously are: 1) The sucrose would be deviated to the formation of defense substances, such as phenols, phytoalexins and others. 2) The sucrose would be deviated to others parts of the plant, in 2l physiological process similar to that of the leaves in senescence. 3) There would be an increase in the respiration and metabolism of the infected tissues, with the implicit consumption of the stored energy.