The isomerization of isobutyraldehyde (IBH) was studied over aluminium-free ZSM-5 zeolite catalysts with different boron content in the framework. The selectivity to methyl ethyl ketone (MEK) is high, ca. 83 wt%. With the increase of boron content, the conversion of IBH increased but the selectivity to MEK decreased. The effects of Na content, cation-exchange, 500 degrees C-steam treatment and temperature on catalyst activity were investigated. It was observed that cation-exchange resulted in the decrease of conversion of IBH and increase of selectivity to MEK, but 500 degrees C-steam treatment resulted in the decrease of selectivity to MEK.