Thermal ring opening of diphosphiranes 1a-g leading to 1,3-diphosphapropenes 2 by preferential P-P bond rupture is investigated. The experimental approach indicates that the regio- and stereoselectivity of the reaction depend on the nature of the ring substituents. For the unsymmetrical functionalized diphosphiranes 1a,b only the trans-syn diphosphapropenes 2a,b are quantitatively obtained whereas for 1c-f the resulting trans-gauche diphosphapropenes are the major products. In addition to the trans diphosphapropenes, the corresponding cis isomers are formed for 1e,f. The theoretical calculations (MNDO anti ab initio) performed on unsubstituted derivatives reveal two preferred mechanisms involving exo-exo biradical or exo-endo radical intermediates. These two mechanisms allow the interpretation of all the experimental results,