Three simple electrolyte systems were tested for determination of four alkali and alkaline earth cations (K+, Na+, Ca2+, Mg2+) in aqueous sulfate solutions. The separation was achieved in a system of 5 mM 4-aminopyridine and 5% v/v methanol with indirect UV detection at 214 nm, in which the electrolyte pH was adjusted to be 4.30 by adding 1 M glycolic acid. Four cations were well separated within 5 min at an applied voltage of 15 kV. Linear relationships of the calibration curves were obtained up to 50 ppm for all four cations. To evaluate this electrolyte system, the determination of these cations was also conducted for real rain waters.