Process is claimed for the prepn. of cyclopentanone (I) by cyclisation of gaseous acidifying acid (II) in an inert atmos. at temps. of 250-450 deg.C in the presence of either oxy-acids or basic oxides as catalysts: The oxy-acid catalysts comprise silicic acids formed by reaction of phosphonic acid (or its monosodium salt) with silica gel, the P cpd. being present at up to 10 wt.%. The basic catalysts are oxides of gp. (I) or (II) on a support e.g. Al2O3. With the basic catalysts injection of water vapour is necessary to promote reaction (although this action may also, opt., be taken with acidic catalysts). The process is carried out in a fluidised bed reactor in the presence of an inert gas e.g. N2 or opt. H2O, the latter used in mol. ratio of 2-10:1 (4-6:1) w.r.t. cpd. (II).
