The surface work function (WF) and substrate temperature dependence of the NaCl thin-film growth on Ag(001) have been studied by noncontact atomic force microscopy and Kelvin probe force microscopy. In the sub-monolayer range, the NaCl film is composed of large crystalline islands, which decrease in density and increase in size with increasing temperature during deposition. Each island is composed of a large base island 2 monolayers (ML) thick (for T > 343 K), which collects impinging NaCl molecules that form ad-islands on top. Kelvin probe force microscopy (KPFM) measurements show a reduction of the silver WF by $\Delta \varphi =\varphi _{\mathrm {Ag}}-\varphi _{\mathrm {NaCl/Ag}}=0.69\pm 0.03\, \mathrm {eV}$ with no dependence on the film thickness (1–6 ML), in agreement with recent theoretical calculations. The previously observed nanometer-sized moiré pattern on islands that are 45° rotated with respect to the silver lattice could be observed in the scanning tunneling microscopy mode. However, no contrast could be obtained in KPFM images.