Opto-thermal transient emission radiometry (OTTER) provides a convenient means for in vivo and in situ monitoring of human or animal skin properties and surface concentrations of topically applied substances, such as drugs and cosmetics. This is illustrated with measurements on the rates of disappearance from skin of a number of commercial sunscreen preparations. The break-up of the physical structure of creams on skin can also be monitored through dynamic phase change phenomena, induced by a laser pulse.