A process for effectively producing a 4-(4-alkylcyclohexyl)benzaldehyde, 4-(cyclohexyl)benzaldehyde, a 4-(trans-4-alkylcyclohexyl)benzaldehyde and a (trans-4-alkylcyclohexyl)benzene useful for electronic material applications such as liquid crystals and for pharmaceutical and agrochemical applications, etc., are disclosed. The present invention provides (1) a process for producing a 4-(4-alkylcyclohexyl)benzaldehyde or 4-(cyclohexyl)benzaldehyde by formylating a (4-alkylcyclohexyl)benzene or cyclohexylbenzene with carbon monoxide, (2) a process for producing a 4-(trans-4-alkylcyclohexyl)benzaldehyde by formylating a (4-alkylcyclohexyl)benzene having a cis/trans molar ratio of 0.3 or less with carbon monoxide, and (3) a process for producing a (trans-4-alkylcyclohexyl)benzene by isomerizing a mixture of the cis and trans isomers of a (4-alkylcyclohexyl)benzene, all of the processes being performed in the presence of HF and BF
3
.