The microemulsion behaviors of europium and sodium salts of naphthenic acid were studied using fluorescence measurements. It was found that 5D0 decay coefficient of europium was 2.08 ms-1 for 100%europium naphthenate, and the coefficients were 1.56, 1.08 and 0.83 ms-1 for 75%, 50% and 20%europium salt of sodium saponified acid respectively. It can be seen that the decay coefficients were also 2.08 ms-1 at the inflexion of curves in the 75%, 50%and 20%europium naphthenate systems with water content. The data reveal that, if there is enough water in the microemulsion, the coordination condition of europium ion in the above microemulsion is similar. It was also found that, for the 75%、60%、50%、40%and 20%europium naphthenate microemulsion systems, the relative fluorescence intensities sharply decrease with the increase of water content and reach a steady values at the same water content.
