Ionically self-assembled monolayers (ISAMs) have recently been shown to spontaneously exhibit a polar ordering that gives rise to substantial second order nonlinear optical response. The deposition of ISAMs has been studied in situ via second harmonic generation. This is a particularly sensitive probe of the growth of nanometer-thick films since the centrosymmetry of the immersion solutions, the substrate, and the container yields no SHG contribution from these bulk components. Upon immersion in the NLO-active polyelectrolyte solution, the SHG rises sharply over the first minute. When a film is immersed into salt water, the SHG decreases significantly only to be restored when the salt solution is replaced with deionized water.