We describe surface tension and neutron and X-ray reflectivity measurements on layers of poly(2-(dimethylamino)ethyl methacrylate-block-n-butyl methacrylate) copolymer (poly(DMAEMA-b-BMA)) (72 mol % DMAEMA) adsorbed at the air-water interface, together with parallel measurements on the statistical DMAEMA-BMA copolymer (90 mol % DMAEMA). The DMAEMA block is a weak polyelectrolyte, and we have studied the properties of these two adsorbed layers as a function of pH and ionic strength. The variation of the surface tension with the three variables, polymer concentration, pH, and ionic strength, is somewhat complex, but combined with the adsorbed amounts determined from the reflection experiments, we have been able to give semiquantitative explanations of this variation using the Gibbs equation. The main factor that must be taken into account is the adsorption (or depletion) of the large number of ions associated with a single copolymer molecule. Different conclusions can he reached about the relative surface activity of the diblock and statistical copolymers depending on whether surface coverage or surface tension is used as the measure of surface activity. The coverage of the two copolymers in terms of the the average number of segments adsorbed is comparable under most conditions, but the surface tension of the diblock is always significantly higher than that of the statistical copolymer. This is the opposite of what is usually supposed, but we argue here that, when the hydrophilic or polyelectrolyte block is the larger block and hence dominates the space occupied at the interface, the hydrophobic block has little effect on the surface tension. However, by ensuring strong adsorption of the copolymer as a whole, the hydrophobic block indirectly causes a large surface concentration of ions, which raises the surface tension to a higher value than would normally be expected. The combination of neutron and x-ray reflectivity shows that there is a thin copolymer layer, typically about 6 Angstrom thick, protruding out of the aqueous surface and a thicker one, typically about 15 Angstrom, immersed in the water. Under all conditions of pH and ionic strength the surface layer remains fairly concentrated (area per average segment less than about 70 Angstrom(2)). When the DMAEMA segments are fully charged and the ionic strength is high, this layer is more extended into the solution.