Using 4 year data of MTOF and PM, two CELIAS sensors flying on SOHO, we report the sulfur abundances in comparison to magnesium and calcium (two low--FIP elements), and for the first time we measure the sulfur isotopic abundance ratio for slow solar wind. During the period in which the proton velocity was 380+-2 km/s we obtain [S]/[Mg]=0.26+-0.03, [S]/[Ca]=4.7+-0.5, and [S 34]/[S]=0.043+-0.009. We compare these measurements with the available measurements reported in literature, and we check the quality of the results by using the magnesium isotopic ratio and the calcium over magnesium abundance ratio as a control. Finally, as further results we also obtain the absolute abundance of the previous elements. In astronomical notation we have: A_S=7.44+-0.04, A_Mg=8.03+-0.05, A_Ca=6.77+-0.04.