Three different silicon oxynitride layers were fabricated by varying NH ₃ /N ₂ O flow rate ratios in low pressure chemical vapor deposition (LPCVD) process. Sodium and aluminum were then coimplanted by implanting sodium ions with the energy of 100 ke V and dose of 5 × 10 ¹⁶ cm ^-2 into an aluminum buffer layer on silicon dioxide and three different silicon oxynitride layers. The composition of the as-deposited silicon oxynitride layers was analyzed by sputtered neutral mass spectroscopy (SNMS). Sodium, potassium and pH-sensing properties of the layers were investigated on an electrolyte-isolator-silicon (EIS) structure using high frequency capacitance-voltage (HF-CV) measurements. Differences of pH, sodium and potassium sensing properties between the as-deposited silicon oxynitride layers and the coimplanted silicon oxynitride layers were investigated. The sodium and aluminum coimplanted layers showed better sodium and potassium sensitivity as well as a lower sensitivity towards hydrogen ions. The effect is more pronounced for higher oxygen concentration in the layers. On the other hand the stability of ion response of the layers, in contrast, is better for the higher nitrogen content of the layers.