Abstract The fluorescence emission of iron-free cytochrome c (0Cyt c) in a glassy matrix was investigated under conditions of very low temperature (4.2 K.) and narrow bandwidth laser excitation. Excitation into the vibronic band, Qx(1,0) resulted in highly resolved emission spectra of linewidth 10-20 cm−1. Using the model of selective excitation developed by Abram el al. (1975) and McColgan et al. (1978), the emission spectra of vibronic excitation afforded a method to investigate excited state vibrational structure. Furthermore, emission profiles have shown that in 0Cyt c, the site distribution (inhomogeneous broadening) has a width in the order of 200cm−1.