The formylation of 2-methylfluorene and of 3-methylfluorene according Rieche yields 2-methyl-7-fluorene-carbaldehyde and 3-methyl-2-fluorenecarbaldehyde respectively. The constitution of the obtained aldehydes is proved by their conversion into the known 2,7-dimethyl-fluorene and 2,7-dimethyl-9-fluorenone, respectively into the known 2,3-dimethylfluorene and 2,3-dimethyl-9-fluorenone.