Photochemical reduction of aqueous Ag+ and [AuCl4](-) into alloy Au-Ag nanoparticles (Au-Ag NPs) with intense laser pulses is a green synthesis approach that requires no toxic chemical reducing agents or stabilizers; however size control without capping agents still remains a challenge. Hydrated electrons produced in the laser plasma can reduce both [AuCl4](-) and Ag+ to form NPs, but hydroxyl radicals (OH center dot) in the plasma inhibit Ag NP formation by promoting the back-oxidation of Ag-0 into Ag+. In this work, femtosecond laser reduction is used to synthesize Au-Ag NPs with controlled compositions by adding the OH center dot scavenger isopropyl alcohol (IPA) to precursor solutions containing KAuCl(4 )and AgCIO4. With sufficient IPA concentration, varying the precursor ratio enabled control over the Au-Ag NP composition and produced alloy NPs with average sizes less than 10 nm and homogeneous molar compositions of Au and Ag. By investigating the kinetics of and [AuCl4](-) coreduction, we find that the reduction of [AuCl4](-) into Au-Ag NPs occurs before most of the Ag+ is incorporated, giving us insight into the mechanism of Au-Ag NP formation.