We present a study of the molecular gas throughout the disk of M 31. From observations with the IRAM 30-m telescope in the HCN and HCO^+ lines we have deduced the distribution of the dense gas in the disk of the galaxy and its variation as a function of the galactocentric radius. We have focused on a segment of a spiral arm to compare ISM tracers (CO, H I), star formation tracers (Hα, 24 μm) and the [C II] line emission from ISO observations. [C II] emission is an excellent tracer of star formation and by comparison with photodissociation region models we have estimated the quantity of H I that is the product of H_2 dissociation.