3,4-Dimethyl-1-thio-1-phenylphosphole reacts, under elevated temperatures, with tropone to give two [4+2] isomeric adducts: Endo- syn-5,6-dimethyl-3-thio-3-phenyl-3-phosphatricyclo [5.3.2.0 2,6]dodec-4,8,11-trien-10-one ( 1) and Endo- anti-5,6-dimethyl-3-thio-3-phenyl-3-phosphatricyclo[5.3.2.0 2,6]dodec-4,9,11-trien-8-one ( 2). In addition to the endo- exo, syn- anti relationship the phosphorus configuration in 1, 2 and some derivatives was also elucidated by means of NMR-shift reagents and 2 J PH coupling constants.