2-Guanidinopurines were prepared as derivatives of 2,6-diamino-9-[2-(phosphonomethoxy)ethyl]-9H-purine (PMEDAP) (1), which shows an important antiviral activity. It completes earlier described synthesis of 6-guanidinopurine derivatives. The title compounds were obtained by the reaction of the corresponding 2-chloropurines with guanidine. 2- And 6-guanidinopurines were used as model compounds for determination of dissociation constants (pKa) of their ionogenic groups by capillary zone electrophoresis. The pKa values of ionogenic groups of the above compounds were compared with those of the corresponding aminopurines. The pKa of guanidino group at the purine moiety varies from 7.77 to 10.32. There is no protonation of N1-position in contrast to aminopurines. None of these compounds showed any antiviral activity.