MeCH=CH(2) was formylated with CO/H(2) mixt. (1:1 to 1.5:1) to aldehydes in an org. solvent in presence of PPh(3) (10-22%), Rh(CO)(PPh(3))[(MeCO)(2)CH(2)] catalyst (Rh content 300 ppm) and Me(2)CHCHO/Me(CH(2))(2)CHO mixt. (10:1) up to 50% at 90 degrees C and 1.75 MPa. No excess of H(2) was used to limit the hydrogenation of MeCH=CH(2) to Me(2)CH(2). The aldehydes were sepd. by distn. outside the reactor and the distn. reso-due was recycled to the reactor. The highest selectivity of Me(CH(2))(2)CHO formation (about 99%) was achieved at CO/H(2) ratio 1.21, initial aldehyde concn. 10% and PPh(3) concn. 16%. The conversion of MeCH=CH(2) was about 44%, the Me(2)CHCHO/Me(CH(2))(2)CHO ratio about 11.