The physical and chemical properties of PAMAM-AT dendrimers' interior were investigated using the fluorescent, solvatochromic probe phenol blue. In aqueous solutions of each generation studied except GO, two discrete dye populations were clearly observed. PAMAM-AT dendrimers were shown to form a tight, nonpolar association with the majority of available dye within the dendrimer interior, probably near its core. In the absorption and steady-state fluorescence emission spectra, a microenvironment of decreasing polarity in increasingly larger-generation PAMAM-AT dendrimers (up to G3) is seen for the associated probe. The remaining larger-generation dendrimers (G4-8) all provide a microenvironment of essentially equal polarity. Fluorescence anisotropy values for phenol blue in the PAMAM-AT dendrimers demonstrate the dye's sensitivity to the changing molecular volumes of the dendrimer generations. Model compounds that mimic PAMAMAT's surface groups and branching moieties were used to better define the associated dye's location. The mimics farther confirm that phenol blue is associated inside the dendrimer, where it does not interact with the dendrimer surface groups.