The present invention relates to ligands and catalyst systems for the hydroformylation of short and long chain olefins, preferably for the hydroformylation of ally alcohol producing 4-hydroxybutyraldehyde. The ligands disclosed herein are all-trans phosphinomethyl-cyclobutane ligands, such as, for example, all-trans-1,2,3,4-tetra[bis-(3,5-xylyl)phosphinomethyl]-cyclobutane. The catalyst systems comprise these all-trans phosphinomethyl-cyclobutane ligands in combination with an organometallic rhodium complex such as, e.g., (acctylacetonato)-dicarbonyl-rhodium (I). The ligands and catalyst systems of the present invention may be employed in the hydroformylation of olefins, in particular in the hydroformylation of allylalcohol, and provide improved selectivity and high reaction yields. wherein R1 is alkyl, preferably methyl, ethyl or propyl, R2 is H or an alkoxy group, R3 and R4, independently of one another, CH2OR1, CH2O-aralkyl, CH2OH,CH2—[P(3,5-R1,R1-4-R2-phenyl)2] or CH2O—(CH2—CH2—O)m—H (with m being an integer between 1 and 1.000).