The He(I) photoelectronspectra of methyl allenyl ether and its sulphur analogue furnish information on lone pair (n)/π conjugation. Evidence is presented that methyl allenyl ether in the gasfase exists in two conformations being for a major part the stable s-cis and to a minor degree the s-trans rotamer. Empirical calculations (MIEHM) substantiate our proposals, whilst a semi-empirical method (MINDO/3) is not in disagreement with it. Our result is in contradiction with previous electron diffraction work.