Preparation of (meth)acrolein and/or (meth)acrylic acid

A process for preparing (meth)acrolein and/or (meth)acrylic acid by heterogeneously catalyzed gas phase partial oxidation by charging a fresh fixed catalyst bed disposed in a reactor at elevated temperature with a charging gas mixture which, in addition to at least one organic precursor compound to be partially oxidized and molecular oxygen as an oxidant, comprises at least one diluent gas which behaves substantially inertly under the conditions of the heterogeneously catalyzed gas phase partial oxidation, which comprises carrying out the process, after the composition of the charging gas mixture has been established, at substantially constant conversion of the organic precursor compound and at substantially constant composition of the charging gas mixture, initially over a startup period of from 3 days to 10 days at a low hourly space velocity and subsequently at a higher hourly space velocity of the charging gas mixture on the catalyst charge.