Three nearby located mires (Svetupe, Dzelve and Eipurs Mire) were selected for this study with the aim to analyze functional groups of low moor and raised bog peat by using Fourier transform infrared spectroscopy. Functional groups of peat and their relations from two ombrotrophic and one minerotrophic mire were studied. Despite small distance between the sites, due to much diverse character of bog development, properties of the mires differ significantly. Studied peat was characterized by radiocarbon dating, loss on ignition analysis, analysis of botanical composition and decomposition degree. Infrared spectra of low moor peat help to identify the four main building structures: alcohols, aliphatic hydrocarbons, carboxylic acids (C=O band) and polysaccharides, while peat from both raised bogs shows also presence of benzene containing structures, alkenes and carboxylic acids. The results show that high decomposition degree of low moor peat simplifies infrared spectra and information of fen peat shows only most persistent structures remained in peat, while FT-IR spectroscopy provides a more detailed picture of raised bog peat.