Simazine (2-chloro-4,6-bis(ethylamino)-s-triazine) was adsorbed from aqueous solutions on three types of amorphous silica, one commercial nonporous and two sol–gel with different porosity. Adsorption takes place on both types of solids in a narrow pH interval around 5.5. This fact, pH changes along adsorption, and IR spectroscopic data concerning dried samples provide evidence that adsorption involves proton transfer from acidic SiOH species to N atoms at ethylamino chains, those at the ring being practically not basic. Two adsorbate species are involved with markedly different interaction enthalpy, causing the presence of two regions in the pseudoisotherms, one irreversibly held, the other partially reversible. Possibilities for the occurrence of two species are discussed. The most likely explanation seems to be a two steps process, in which first a protonated molecule is formed, onto which a second molecule may anchor so forming a dimer, held together by either sharing of the positive charge or hydrophobic interactions. The available surface appears to have basically the same properties in all three cases. Microporosity, however, introduces diffusional constraints and limits the extent of surface available to adsorption.