3,4-methylenedioxymethamphetamine (3,4-MDMA, "Ecstacy") and its 17 isomers and isobaric substances are studied using liquid chromatography (LC)-positive electrospray ionization-mass spectrometry (MS). 3,4-MDMA is a controlled substance, whereas in many countries the other studied isobaric compounds are not. A method for confirmation of the presence of 3,4-MDMA in drug seizures is developed and validated. Using single MS, the compounds produce an intense protonated molecule and some characteristic fragments; but tandem MS (MS-MS) is applied to enhance specificity. The MS-MS fragmentation is studied in order to distinguish 3,4-MDMA from the other 17 related compounds. However, the MS-MS spectra of 3,4-MDMA and six related compounds are very similar. Therefore, the LC-MS-MS method is developed for the unambiguous identification of 3,4-MDMA. The use of a monolithic column allows for 5-min gradient runs. This qualitative method is tested with 49 Ecstacy samples seized by the police. All results are congruent with the ones obtained with other methods.