Based on the principle of the Ion Selected Electrode dilution method (indirect), a complete set of reagents for determining Na, K, Cl and CO2 have been developed. Compared with original BECKMAN reagents being applied in the SYNCHRON EL-ISE analyzer, the self-made reagents may satisfy clinical requirements for precision and accuracy, and have a good correlation with original reagents: rNa = 0.998, rK = 0.999, rCl = 0.998, rCO2 = 0.889, and the CV% within and between batches are less than 3.3 and 5.7 respectively; the rate of recovery is 99%-105%. NO influence on the electrode has been found through observing electrode slopes and SPAN values.