A hydroformylation process for preparing 3,5,5-trimethylhexanal comprising reacting 2,4,4-trimethylpent-2-ene with H2 and CO in a reaction zone in the presence of one or more free organophosphite ligands of the general formula (I) wherein R1, R2, R3, R4 and R5 are each independently H, C1- to C9-alkyl or C1- to C9-alkoxy and R1, R2, R3, R4 and R5 are not H at the same time, and R6, R7, R8, R° and R1° are each independently H, C1- to C9-alkyl or C1- to C9-alkoxy and Re, R7, R8, R° and R1° are not H at the same time, and R11, R12, R13, R14 and R15 are each independently H, C1- to C9-alkyl or C1- to C9-alkoxy and R11, R12, R13, R14 and R15 are not H at the same time, and a homogeneous rhodium catalyst complexed with one or more organophosphite ligands of the general formula (I) at a pressure of 1 to 100 bar abs and a temperature of from 50 to 200° C.