Moist air flows with and without consideration of inlet wetness and assuming fogging were numerically investigated in a transonic compressor of an industrial gas turbine, and the causal relationship between the wetness and the deposition of water droplets on the blade surface was clarified. First, we simulated three-dimensional moist-air flows through the 1.5-stage rotor and stator blade rows in the transonic compressor of an industrial gas turbine operating at a Tokyo Electrical power plant, considering variations in the inlet wetness and the number of aerosol particles. The results indicated that the shock generated in the first-stage rotor passage was the primary factor in the evaporation of the moist air, decreasing its temperature, whereas the wetness was highly dependant on the deposition of water droplets on the blade surface. We further simulated two-dimensional humid-air flows through a transonic compressor cascade channel under a number of conditions, varying the aerosol particle size, number of aerosol particles, humidity, blade chord length and pressure ratio. The results indicated that the condensation of humid air was captured locally in the supersonic region, while the deposition of the condensed water droplets was mainly influenced by the first three factors. Overall, this study indicates that wetness and deposition are in a trade-off relationship with regard to the performance of gas turbines.