Different aliphatic and aromatic aldehydes such as isobutyraldehyde, ethanal, n-propanal, n-butanal, n-hexanal, n-octanal, and substituted benzaldehydes were cyclotrimerized into their corresponding 2,4,6-trialkyl-1,3,5-trioxanes in the presence of commercial bulk zinc oxide at room temperature under neat conditions within a short span of time with high yield and excellent selectivity. Liquid products were formed as a separate phase and were decanted easily, whereas solid products were isolated by simple extraction. Effect of different additives, mol % of catalyst, kind of substrate, and reaction temperature were examined on the progress of cyclotrimerization reaction.