Temperature-programmed desorption (TPD) was used to study the desorption and photochemistry of trans-stilbene on Al2O3(0001) in ultrahigh vacuum. At low coverages, the TPD traces of cis-stilbene and trans-stilbene both show only one peak, attributed to monolayer desorption, at 275 and 315 K, respectively. As the coverage is increased, a second, multilayer peak is seen at 230 K for cis-stilbene and at 260 K for trans-stilbene. The relative intensities of the monolayer and multilayer peaks were observed to fit a statistical adlayer sticking model similar to those of other hydrocarbons and described in previous work. When trans-stilbene is exposed to light at 313 nm, TPD traces show a new peak at 380 K which is attributed to the formation of the stilbene dimer, 1,2,3,4-tetraphenylcyclobutane (or isodistilbene). The photodimerization is coverage and temperature dependent with little or no photochemistry occurring at coverages below 0.10 ML or temperatures below 100 K. The data are consistent with the dimerization occurring through an excimer intermediate. Little or no photoisomerization was observed at any of the coverages studied.