The UV-vis spectra of holmium atoms isolated in argon and krypton matrices at 9 K have been studied in the 200-800 nm region. By comparison with gas phase data it is possible to separate the multitude of absorptions in the 400 nm region into transitions belonging to holmium atoms in two different sites, only one of which is photostable. Prolonged irradiation of dilute Ho/Ar matrices results in the simultaneous growth of two new doublets at 498/504 and 558/562 nm, attributed to the holmium dimer Ho-2. Cocondensation of holmium atoms with argon at temperatures above 10 K results in gradual breakdown of isolation conditions with indications of global clustering. Not even traces of isolated holmium atoms can be observed at deposition temperature above 17 K. Photoactivation permits tentative assignment of bands at 608 and 450 nm to Ho-3 and Ho-4, respectively.