The isotopic composition of ozone depleting methyl halides may provide valuable information on the sources and sinks of these compounds. However, so far mostly stable carbon isotope analysis of methyl chloride and methyl bromide (CH₃Br) has been attempted. Especially halogen isotope analysis has been hindered by the challenge to obtain sufficiently large amounts of methyl halides to meet the detection limits of existing isotope analytical methods. The purpose of this doctoral thesis was to develop both a high-volume cryogenic collection system for methyl bromide (Article II) and an analytical technique being able to analyze the sampled amounts of CH₃Br for its Br isotopic composition, which was accomplished by using gas chromatography multiple-collector inductively-coupled plasma mass spectrometry (Article I). These methods were applied in the field campaign from which we report the first bromine isotopic values of CH₃Br in the atmosphere (Article III), being in the range of -0.47 to + 1.75 ‰ vs. SMOB (Standard Mean Ocean Bromide). A laboratory study on pectin and halophyte plant material (Article IV) gave an insight in Br isotope composition of abiotically formed CH₃Br which may be a main source to the atmospheric budget. These plant experiments yielded δ⁸¹Br values of 0 to -2 ‰ SMOB. Atmospheric CH₃Br and this potential source showed partly distinct δ⁸¹Br ranges and demonstrate the potential of Br isotopes for source apportionment.