A simple non-invasive ultra-violet/visible (UV/vis) diffusive reflectance spectroscopy combined with fiber-optics was investigated to elicit the dynamics of skin penetration in vivo of a pharmaceutical, aminolevulinic acid polyethylene glycol cream (5-ALA-PEG cream). Temporal data of the reflectance, R(lambda), were measured from a bare skin region and from a skin region treated with 5-ALA cream. The difference in apparent optical density [(DeltaAOD)=Deltalog[1/R(lambda)]] between the treated and untreated regions of the skin was gradually increased to a local maximum value with an increasing time of measurement, and then it was decreased in an oscillatory manner. This temporal increase in DeltaAOD was interpreted as a diminution of total reflectance by an increased scattering inside the skin tissue as topically applied materials penetrated gradually and temporally accumulated into the skin. A following decrease in DeltaAOD was attributed to diminished scattering effects as the penetrated materials gradually cleared out by cellular uptake of 5-ALA and circulation by epidermal capillary. In conclusion, it was possible to measure the skin absorption kinetics of the topically treated materials, including the penetration time and clearing out from the temporal course of the differential AOD.