The rate of Sn3F3PO4 formation subsequent to in vitro SnF2 interaction with human dental mineral has been investigated using light microscopy, surface replicas and infrared internal reflection spectroscopy. It was found that formation of Sn3F3PO4 is a time-dependent phenomenon. Further, it was hypothesized that Sn3F3PO4 crystals are probably not a significant factor with regard to the clinical efficacy of topical SnF2 treatment of sound dental tissues.