An ebulliometer was used to measure total vapor-pressure (PTx) data on nine mixtures of water + N-methylethylenediamine (and the pure components) between 52 and 116 degreesC. Bubble-point data were measured at six pressures from 13.33 to 101.325 kPa. These PTx data indicate the following azeotropic behavior: at pressures below 32.7 kPa there is a single maximum-boiling azeotrope, above 46.7 kPa there is no azeotrope, and between 32.7 and 46.7 kPa there is the possibility of a double azeotrope. This type of double azeotrope is rare because the pure component vapor pressures are considerably different (P-sat1 / P-sat1 approximate to 1.6); it may be that the apparent extrema in the Tx data are due to artifacts related to the purity of the N-methylethylenediamine (approximate to 95 mass%). A Redlich-Kister G(E) model was fitted to isotherms at 60, 85, 90 and 100 degreesC via Barker's method with an average standard error of 0.52% in pressure. The system exhibits large negative deviations from ideality (derived gamma (infinity) = 0.05-0.67) which decrease with increasing temperature. Equimolar G(E) IT values thus derived increase with increasing temperature which predicts a negative H-E. Equimolar C-P(E) data, measured by differential scanning calorimetry (DSC), are positive and also increase with increasing temperature. These mixture thermodynamic data show that the system water + N-methylethylenediamine belongs to the class of mixtures where G(E) < 0, H-E < 0, and TSE < 0. Therefore, the data indicate that (1-2) hydrogen-bonding of water with N-methylethylenediamine is greater than either the (1-1) or (2-2) hydrogen-bonding in the pure components.