Surface functionalization has numerous applications worldwide. Silicon oxide has been a research material of choice. However, tin dioxide (SnO2) films are employed in many applications especially in gas sensors, and little studied in regard to functionalization. Thus, they were chosen to be functionalized via 3-aminopropyltriethoxysilane (APTES). Different synthesis parameters were tested such as APTES grafting by vapor or liquid phases deposition. In liquid, many parameters were investigated: water presence, reaction times, and APTES concentration. The presence and reactivity of grafted amine-terminated film on SnO2 were carried out by Alexa Fluor(A (R)) molecules. In addition, APTES grafting was characterized using attenuated total reflectance Fourier transform infrared spectroscopy and X-ray photoelectron spectrometry techniques. These characterizations showed how synthesis parameters affect the amount and thickness of APTES films. Optimal liquid silanization parameters were determined in order to obtain a saturated SnO2 surface with APTES molecules. Importantly, the addition of 5 vol% H2O to the APTES solution provided denser surface coverage, by hydrolyzing the ethoxy groups to silanol. An almost 50% improvement over anhydrous liquid and vapor methods was obtained.