The present invention relates to the field of medical chemistry, particularly to the process for preparation of 4-(3-bromo-4-oxobutyl)- benzoic acid. Pharmaceutically active compound disodium salt of N-[4-[2-(2-amino-4,7-dihydro-4-oxo-1H-pyrrolo[2,3-d]pyrimidin- 5-yl)ethyl]benzoyl]-L-glutamic acid could be prepared in multistep synthesis and one of intermediates is 4-(3-bromo-4-oxobutyl)- benzoic acid methyl ester (BROMOPRODUCT), commonly pre¬pared by bromination of 4-(4-oxobutyl)-benzoic acid methyl ester (ALDEHYDE). This invention provides process for bromination of ALDEHYDE using in complex included bromine, particularly pyridinium bromide perbromide, characterised in that provides to obtain BROMOPRODUCT in simple process with high yield of end-product.