Plausible prebiotic conditions for the phosphorylation of nucleosides by inorganic phosphate were reported by Lohrmann and Orgel in 1971. This reaction was carried out on heated dry films and promoted by urea. The major products formed were nucleoside-2':3' cyclicPs; 5'-NMPs and other derivatives were also formed. Minor modifications of the Lohrmann and Orgel system have resulted in the preferential formation of 5'-NMPs. In this modified system a 2-fold preference for phosphorylation of the 5'-OH group over the 3'(2')-OH group was observed and the formation of other derivatives was minimized. The small amounts of bis compounds that were formed in this system could be quantitatively removed by selective binding to the mineral hydroxylapatite at moderate ionic strengths. It was also discovered that under hydrolytic conditions there was a 3:1 preference for removal of phosphates attached to the 3'-OH group over the 5'-OH group. A recycling procedure for obtaining additional 5'-NMPs from bis compounds and 3'-NMPs is proposed.