Densities of solutions of 1,3-dimethylurea and tetramethylurea in H2O and D2O have been measured at 15, 25, and 40 degrees C over wide concentration ranges using a vibrating tube densitometer to obtain apparent (V-phi) and partial (V-2) molar volumes. A comparison of the data with those for urea shows interesting differences. While for urea V-2(E) is positive, monotonously increases with concentration, and displays an inverse isotope effect, V-2(E)(H) - V-2(E)(D) < 0, <0 V-2(E) values for the methylureas are negative, go through a minimum, and show positive solvent isotope effects which are smaller than those for typical inorganic electrolyte solutions.