The validity and the accuracy of both the Owens and Wendt and the Lifshitz-van der Waals/acid-base (LW/AB) methods for the determination of surface tensions of solids have been examined for a wide variety of situations. In each case, the allowed range of contact angles that result in positive values of all the square roots of the surface tension components of the solid has first been determined. Then the maximum relative errors in the surface tensions of solids that result from errors in contact angle measurements have been calculated within the allowed range. For both methods, it has been found that the maximum relative errors are minimal if one of the liquids is apolar. In the case of the LW/AB method, minimal errors are obtained if, in addition, the other two liquids are monopolar with different polarities. However, the more similar are the properties of the liquids, the narrower is the allowed range, and the larger are the maximum relative errors. (C) 2003 Elsevier Science (USA). All rights reserved.