The magnetoresistance (MR) effect of the low-temperature phase of silver selenide (α-Ag2+δSe) is measured as a function of composition. Very small composition variations in the order of Δδ=10-6 are achieved by coulometric titration and can be performed simultaneously during the MR measurement. A homogeneous Ag2+δSe shows an ordinary magnetoresistance (OMR) effect, which can be well described by the two-band model. For silver selenide with a heterogenous silver excess, we found quite a different MR behavior. Up to a minor silver excess of 1×10-4≪δ≪1×10-2, a saturating negative MR effect, a linear positive MR effect, or a superposition of both can be measured. The microstructure of the silver-rich Ag2+δSe determines its complicated MR behavior. A heterogeneous silver selenide with a larger silver excess (δ≫10-2) shows again an OMR effect.