An investigation of the usefulness of the microwave oven for water content determination and for drying of soil before testing was performed. Soils ranging from highly plastic clays to clean sand were used. The influences of initial water content, amount of soil, and soil type on the drying time were investigated. Differences between water content determined using a microwave oven and a conventional oven decreased with decreasing plasticity of the soils. The microwave oven can be used for quick drying of soil, but accurate determinations of water content may only be achieved for nonplastic soils such as coarse silts, sands, and gravels. The change in classification according to the Atterberg limits was taken as a measure of the effects of heating by microwaves. The plasticity and swelling potential of the soils decreased after preheating in the microwave oven, and these effects are comparable to the effects of drying the soils in a conventional oven at high temperatures.