Abstact: —We study spin-valley relaxation dynamics in two-dimensional Mo_xW_1–xSe₂/WSe₂ heterobilayers with different relative Mo/W concentration x in the monolayer alloy. Three types of heterobilayers with x = 1.00, 0.50, 0.33 are studied in time-resolved Kerr rotation experiments for different wavelengths and temperatures. The spin-valley relaxation times are found to decrease from ~10 nanoseconds for x = 1.00 to ~50 ps for x = 0.33. The observed relaxation times are limited by the recombination of indirect excitons formed in the heterobilayers. Our results demonstrate that spin-valley relaxation in alloy-based van der Waals heterostructures can be controlled via their chemical composition.