The 33 Na β decay was studied online using mass separation techniques and a first description of the level structure of the neutron-rich isotope 33 Mg , with N = 21 , has been obtained. The experiment involved the measurement of β-γ, β-γ-γ, and β − n − γ coincidences as well as neutron spectra by time-of-flight technique. The first low energy level scheme for the daughter nucleus 33 Mg is given with five bound states. Spin and parity assignments are proposed according to β feedings and γ-ray multipolarities. β-strength distribution is evaluated, taking into account 1 n - and 2 n -emission channels and it is compared with the calculated GT strength distribution. In particular, the 1 p − 1 h and 2 p − 2 h excitations are shown to result in substantial contribution to the low energy configurations. Allowed β branch to the 33 Mg g.s. [ log f t = 5.27 ( 26 ) ] gives direct evidence for the inversion of ν ( f 7 / 2 ) and ν ( d 3 / 2 ) states.