Stable PtAu alloy colloids with a wide range of compositions were prepared using pulsed laser ablation on single metal-mixture targets in water. The concentration of Pt in the alloys can be tuned by varying the Pt/Au ratio in the targets, which are made by compression molding a mixture of Pt and Au powders at different ratios. Such fabricated PtAu alloy nanoparticles (NPs) show a face-centered cubic structure, and their composition basically follows that of their corresponding targets. The effect of aqueous solution pH and ablating laser fluence on the formation and structure of alloy NPs was further investigated. It is found that PtAu alloy colloids of identical composition can be achieved over a pH range extending from 4.0 to 11.0 and at fluences varying from 4 to 150 J cm^–2 as long as the targets of the same composition are used. This finding suggests that alloy formation is essentially insensitive to both factors in certain ranges, and the method developed herein for the alloy NP formation is quite robust. Moreover, the surface composition, estimated from electrochemical measurements, is identical to the overall composition of the NPs estimated from Vegard’s law and X-ray diffraction data, which is a strong indication of the uniform composition on the surface and in the interior of these alloy NPs.