The first reaction spectroscopy on the ground state structure of [superscript 33]Mg through the measurement of the longitudinal momentum distribution from the one-neutron removal reaction using a C target at 898A MeV is reported. The experiment was performed at the FRS, GSI. The distribution has a relatively narrow width (150 [plus or minus] 3 MeV/c (FWHM)) and the one-neutron removal cross-section is 74 [plus or minus] 4 mb. An increased contribution from the 2p[subscript 3/2] orbital is required to explain the observation showing its lowering compared to existing model predictions. This provides new information regarding the configuration of [superscript 33]Mg and the island of inversion.