Data of water activity in sugar solutions (of sucrose, glucose, fructose, and maltose) were obtained experimentally utilizing a dew point device (Aqua-Lab CX-2) at temperatures of 25, 30, and 35 degrees C, and atmospheric pressure. The thermodynamic models of group contribution (UNIFAC, ASOG) and of equations of state (EOS PR/Classic; EOS PR/SV) were used for modeling and predicting water activity in these solutions. A single set of parameters was obtained for the temperatures tested by fitting the UNIFAC and ASOG models to the experimental data; equations of state were used to determine binary interaction parameters for water-sucrose, water-glucose, water-fructose, and water-maltose. Water activity was predicted for a quaternary mixture (sucrose, glucose, fructose and water), with average relative deviations of 0.5% and 1.3% for the EOS PR/SV and EOS PR/Classic, respectively. For the group contribution methods, the deviations were 0.48% for the ASOG and 0.52% for the UNIFAC.