A conceptually novel method has been developed for determining the three-phase contact angle of solid colloid particles adsorbed at the air-water or oil-water interface. The method is applicable for particle diameters ranging from several hundred nanometers to several hundred micrometers. This new "gel trapping technique" (GTT) is based on spreading of the particles on an air-water or oil-water surface and subsequent gelling of the water phase with a nonadsorbing polysaccharide. The particle monolayer trapped on the surface of the gel is then replicated and lifted up with poly(dimethylsiloxane) (PDMS) elastomer, which allows the particles embedded within the PDMS surface to be imaged with high resolution by using a scanning electron microscope (SEM). The position of the particles with respect to the PDMS surface has been determined from the SEM images, which gives information on the particle contact angle at the air-water or the oil-water interface. Three samples of latex and silica particles of different size and surface chemistry have been examined by using the GTT. It has been found that the contact angles of sulfate polystyrene latex particles at the air-water and decane-water interface determined by the GTT are slightly lower than the contact angles of water drops on flat polystyrene substrates under air or decane.