A dimensional analysis of laser surface alloying (LSA) using measurable experimental variables and known thermophysical properties has been carried out to identify useful dimensionless parameters influencing LSA. Measured solute contents from a two-level factorial experiment alloying nickel into AISI 1020 steel substrates are plotted against these parameters and found to vary hyperbolically. Similar trends are shown to be contained in published data alloying chromium into 1020 steel. The two parameters make good physical sense, point to mechanisms in LSA, and may be used for process control. One parameter is the laser surface alloying parameter (LASAP), where LASAP = P/pud2, and P is incident laser power (W), p is surface gas pressure (N/m2), u is traverse speed (m/s), and d is beam diameter (m). The other is the laser processing parameter (LAPP), where LAPP = Pd/(pau2d2) and p, d, u are as above, whilep is the substrate density (kg/m3) and a is the substrate thermal diffusivity (m /s).