Activated mononucleotides oligomerize in the presence of montmorillonite clay to form RNA oligomers. In the present study, effects of salts, temperature and pH on the clay-catalyzed synthesis of RNA oligomers were investigated. This reaction is favored by relatively high concentration of salts, such as 1 M NaCl. It was shown that the presence of divalent cations was not required for this reaction. High concentrations of NH4+ and HCO3- and 0.01 M HPO42- inhibit the reaction. The yields of RNA oligomers decreased as the temperature was raised from 4 °C to 50 °C. A5′ ppA was the major product at pH's below 6. The catalytic activity of a variety of minerals and three meteorites were investigated but none of them except galena catalyzed the oligomerization. ATP was generated from ADP but it was due to the presence of HEPES buffer and not due to the minerals. Meteorites catalyzed the hydrolysis of the pyrophosphate bonds of ATP. The results suggest that oligomers of RNA could have formed in pH 7–9 solutions of alkali metal salts in the presence of montmorillonite clay.