4-methyl-4-phenyl-1-pentanals of the general formula I ##STR1## where R.sup.1 is hydrogen or C.sub.1 -C.sub.4 -alkyl and R.sup.2 is hydrogen or methyl, with the exception of 4-methyl-4-phenyl-1-pentanal, and the use of the methylphenylpentanals of the formula I for imparting fragrance properties to, or improving or modifying the fragrance properties of, perfumes and perfumed products, and the preparation of these compounds by hydroformylation of the corresponding 3-methyl-3-phenyl-1-butenes of the general formula II ##STR2## and, if required, subsequent reaction with formaldehyde and partial hydrogenation.