The addition of linear poly(acrylic acid) (PAA) to a disordered low molecular mass poly(ethylene oxide) poly(propylene oxide) poly(ethylene oxide) triblock copolymer host (PEO-b-PPO-b-PEO) is known to induce an ordering transition. Small-angle neutron scattering (SANS) measurements are presented to quantify the spatial distribution of the linear homopolymer additive within the ordered microstructure by using deuterium-labeled block copolymers. The analysis, however, requires revisiting the conventional algorithm of expressing the scattering intensity as a product of a form and structure factor, where the two terms are independent, and an alternate formalism is introduced that couples these terms. The SANS data are consistent with a lamellar morphology, but with the unusual caveat of a weak first-order scattering peak relative to a strong second-order peak. This necessitates the use of a four-layer structure for the lamellar repeat unit where the center of the PEO domain is enriched with PAA. The SANS data are fit to quantify the compositional profiles of the three components across the four-layer lamellar repeat unit. As the concentration and/or molecular mass of the PAA additive increases, PEO-PAA-rich and PPO-rich domains gradually emerge. However, it is not until relatively high PAA loadings that compositionally pure PPO domains appear. While the PAA additive is known to form hydrogen bonds with the PEO segments, the measurements suggest that the unfavorable interactions between the PAA and PPO are also important for driving the self-assembly process.