Several properties of the M 31 disk , namely : opacity , extinction law and gas-to-dust ratio are studied by means of optical and near-infrared photometry of ten globular clusters and galaxies seen through the disk . The individual extinctions of these objects were estimated with respect to several sets of theoretical spectral energy distributions for globulars and galaxies . Seven targets are consistent with reddened globulars , two - with starburst galaxies and one - with an elliptical . The extinction estimates agree with semi-transparent disk ( \tau _ { V } \lesssim 1 ) in the inter-arm regions . The total-to-selective extinction ratio in those regions 2.75 \pm 0.1 is lower on average than the typical Galactic value of R _ { V } =3.1 . We also obtained a gas-to-dust ratio , similar to that in the the Milky way . It shows no correlation with the distance from the M31 center .