AbstractThe reaction of p‐cymene with tert.amyl alcohol or methylbutenes in the presence of sulphuric acid or with tert.amyl alcohol or tert.amyl chloride in the presence of aluminium chloride according to the Friedel Crafts method, has been found to yield a polymethylindan mixture consisting of 41% of 1,1,2,3,3,5‐hexamethylindan (XXI), 23% of 3‐ethyl‐1,1,3,5‐tetramethylindan (XX), and 12% of 1,1,3,3,5‐pentamethylindan (III). In addition, 1,2,3,3,5‐pentamethylindan (II) and 1,1,2,3,5‐pentamethylindan (I) have been synthesized.Introduction of an acetyl group into the aromatic nucleus of the first four indans results in indanyl methyl ketones having a musk odour, acetyl‐1,1,2,3,3,5‐hexamethylindan (XXVIII) being the most important of these.