The effect of drop (bubble) size on contact angle was examined over a wide range of drop (bubble) sizes for the CCl4-toluene solution/NaCl aqueous solution/quartz, ethylene gtycol/CCI4-toluene solution/quartz, bitumen/ Na5P3O10 aqueous solution/quartz, water/kerosene/quartz, heptane/water/quartz, and air bubble/water/polyethylene systems. For most systems, a nonlinear relationship between cosine of the contact angle and reciprocal of drop (bubble) base radius was obtained. All known factors affecting the contact angle/drop size relationship are discussed. It was found that the effect of gravitational forces as well as solid strain in the vicinity of the three-phase contact line are not responsible for the contact angle/drop size relationship for the systems examined. It appears that surface heterogeneities (contaminants) have the greatest effect on the contact angle/drop size relationship as is evident for these results from our laboratory. Also, surface roughness can contribute to the effect of drop size on contact angle. A theoretical background is presented and modifications of both the Cassie Equation and Wenzel Equation are derived. A linear correlation of cosine of the contact angle vs. reciprocal of drop base radius was obtained for the heptane/waler/quartz systems, when high purity single-component liquids and a smooth, chemically cleaned, quartz plate were used. The line tension was determined for this system to be (5.1 ± 4.3) 10 − 9N.