The governing thermodynamics of the Madden-Julian Oscillation (MJO) are examined using sounding and reanalysis data. On the basis of four objective criteria, results suggest that the MJO behaves like a moisture mode –a system whose thermodynamics is governed by moisture– only over the Indian Ocean. Over this basin, the small effective gross moist stability, i.e., a slow moist static energy (MSE) export by convection, and slow convective adjustment timescale allows moisture modes to exist for all zonal wavenumbers except 1. Elsewhere, the faster-propagating wavenumber 1-2 components are more prominent and the effective gross moist stability is higher, preventing weak temperature gradient (WTG) balance to be established and causing temperature and moisture to play similar roles in the MJO’s thermodynamics.