A heat treatment was performed on selected epidote and clinozoisite crystals to establish nature of any changes in optical and crystal-chemical properties and to identify a breakdown product using a wide spectrum of analytical methods. Natural samples were heated from 400 to 1200°C at atmospheric pressure in ambient oxidation conditions for 8 hours. Epidote and clinozoisite were stable up to 900°C; those heated at 1000°C, 1100°C and 1200°C exhibited signs of breakdown with cracks and fissures developed. Phase transformations occurred in samples heated at more than 1000°C inducing changes in their colour and clarity. The decomposition of epidote is controlled by probable reaction: 4 epidote/clinozoisite → 2 pyroxene + 2 wollastonite + 4 anorthite + 2 H2O. All breakdown products are well documented by the XRD, Raman spectroscopy, TEM, HRTEM, and EDS. A significant proportion of tetrahedral Fe3+ was documented by the Mössbauer spectroscopy. Epidote-group minerals including allanite-(La), mukhinite and clinozoisite from the Čierna Lehota occurrence reveals a unique complex V, Cr, Mn and REE enrichment, known from few worldwide occurrences. The composition of epidote-group minerals is controlled by REEFe2+(CaAl)-1, REEMg(CaAl)-1, REEMn2+(CaAl)-1, and REEFe2+(CaFe2+)-1 substitutions introducing REE along with VAl-1 and CrAl-1 substitutions. Some crystals have complete zoning sequence with the oldest REE-rich allanite-(La) to mukhinite (Aln-Muk) rimmed by V- and Cr-rich clinozoisite to mukhinite with parts attaining close to allanite-(La) composition (Czo-Muk) and V- and Cr-poor clinozoisite. The compositional variability is caused by variable fluid and rock composition and multistage evolution. Comparison of high-pressure and high-temperature data for clinozoisite showed that a given increase in pressure produced structural effects not similar to those seen after a proportional increase in temperature. Key words: epidote, clinozoisite, solid solution, breakdown products, experimental study