Cu2+-doped K3H(CO3)(2), Rb2CO3, and Rb2KH(CO3)(2) single crystals were investigated by electron paramagnetic resonance (EPR) spectroscopy. The EPR spectrum of K3H(CO3)(2) indicates two different sites for Cu2+ at room and at low temperatures. But the signals for the two sites overlap at 318 K which is attributed to a disorder phase transition. Each of the other compounds exhibits one site. The Cu2+ ion seems to Substitute the K+ and Rb+ ions and the charge compensations are fullilled by the proton vacancies in K3H(CO3)(2), and another K+ and Rb+ in the other Compounds. The spin Hamiltonian parameters g and A for each Compound are determined and discussed.