Partially collective Thomson scattering measurements have been performed on a triggered vacuum arc in tin vapor, which is a candidate source of extreme ultraviolet light for application in semiconductor lithography. In this paper, results on the electron densities and temperatures are presented for the prepinch phase of the discharge. Electron densities and temperatures increase from 1 x 10(23) m(-3) to 1 x 10(24) m(-3) and from 5 eV to over 30 eV, respectively, in about 100 ns. The results are confirmed by Stark broadening data.