The optimum conditions for the best performance of magnesium-selective PVC membrane electrodes based on 1,2-bis(di-arylphosphine oxide)benzene as a new class of neutral carriers were investigated. The most favourable sensor was 1,2-bis(di-tolylphosphine oxide)benzene, especially when an anion excluder, potassium tetrakis(4-chlorophenyl)borate, was included in 2-nitrophenyl octyl ether for ion-selective electrode membrane construction. The electrode showed excellent properties. It gave a linear response with a nernstian slope of 30 mV decade 1 within the concentration range 0.1-3.2 X 10(-5) M MgCl2. The selectivity and response time of the electrode were studied and it was found that the electrode exhibited good selectivity for magnesium in comparison with alkali, alkaline earth and some transition metal ions. The electrode response was stable over a wide pH range. The lifetime of the electrode was about 30 days.