This paper is concerned with a detailed study of the critical state parameters and peak stress envelopes for undisturbed samples of Bangkok Clays (‘Weathered Clay’, ‘Soft Clay’ and ‘Stiff Clay’) taken from a site called Bangpli which is about 28 km from Bangkok. Both triaxial compression and extension tests were carried out with a wide variety of applied stress paths in a manner similar to that adopted by Parry (1960). For all types of tests the end points are presented in (q, p), (w, log p) and (w, log q) plots; q is the deviator stress, p is the mean normal effective stress and w is the water content. The Mohr Coulomb strength parameters are also evaluated for each series of tests. The results are discussed in relation to the critical state concept of Roscoe et al. (1958) and the water content-strength relationships of Henkel (1959).