The cation exchange capacity (CEC) of the special montmorillonite clay SCa-3 has been reported in the literature as 153 meq/100 g. Results of experimentation in this laboratory do not coincide with this published value. The CEC was examined using several different methods including x-ray diffraction (XRD), ammonium exchange/ammonia electrode (Busenberg/Clemency method), solution calorimetry, and combustion analysis. The results of XRD of porphyrin exchanged SCa-3 were inconclusive and indicated either a significantly smaller CEC (~100 meq/100 g) or an unexpected arrangement of porphyrin molecules. A standard method of CEC determination using ammonium exchanged clay and an ammonia electrode also indicated a smaller CEC of 102 meq/100 g. The determination of the enthalpy of hydration of cationic SCa-3 species (sodium, potassium, magnesium, and calcium) also resulted in a CEC of approximately 105 meq/ 100 g. A final combustion analysis of tetraalkylammonium exchanged SCa-3 resulted in a CEC of 105 meq/100g. The reported value of 153 meq/100 g is questionable as the results of four completely different independent analyses all indicate a CEC of approximately 105 meq/ 100 g.