Ultrahigh vacuum scanning tunneling spectroscopy (UHV-STS) is used to make the first in situ measurements of reactant charge transfer with the host basal planes during the early events in lamellar intercalation processes to clarify the role and importance of charge transfer in intercalation reactivity. The level of basal-plane charge transfer is determined from the Fermi level shift relative to the host (TiS2) conduction-band edge. Prior to intercalation, the model reaction of ammonia with lamellar TiS2 exhibits low-level, pressure-independent adsorbate (ammonia) charge transfer to the basal planes, identifying a preintercalation state in which adsorbate charge transfer plays a key role in intercalation reactivity. Such a state can explain the anomalously-high reactivity exhibited by other molecular intercalation systems, such as hydrazine/TiSe2, where available evidence suggests strong preintercalation adsorbate-host charge transfer. Once intercalation occurs, ammonium forms and cointercalates with ammonia via the bulk redox rearrangement intercalation process, with substantial additional basal-plane charge transfer. Such behavior suggests that pre- and postintercalation charge transfer mechanisms can differ fundamentally for the same molecular reactants. UHV-STS has been shown to be a powerful technique for determining the level of basal-plane charge transfer throughout the intercalation process.