Moderate changes in the concentration of poly(sodium 4-styrene sulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) used in fabricating polyelectrolyte multilayers (PEMs) have been found to strongly affect the formation of defects (i.e., gaps) within these films, as revealed by gas permeation selectivity measurements. Thus, PEMs made from 14 alternating layers of PSS and PAH under "high" concentration conditions (15.0 mM in repeat units) exhibit He/N-2 permeation selectivities that are greater than 200. In sharp contrast, analogous films of similar thickness that are made from 24 alternating layers of PSS and PAH under "low" concentration conditions (1.0 mM in repeat units) showed no significant He/N-2 permeation selectivity, reflecting the presence of defects. Analogous PEMs that were fabricated from 14 alternating layers of PSS and PAH, using 1.0 mM PSS and PAH in the presence of added NaCl, where reduced viscosities and polymer conformations were matched with those found under high concentration conditions, exhibited the same thicknesses and very similar barrier properties.