Determining how analytes are sequestered into thin films is important for solid-state sensors that detect the presence of the analyte by oxidative luminescence quenching. We show that thin (230 +/- 30 angstrom) and thick (750 +/- 50 angstrom) films of a first-generation dendrimer comprised of 2-ethylhexyloxy surface groups, biphenyl-based dendrons, and a 9,9,9',9'-tetra-n-propyl-2,2'-bifluorene core, can rapidly and reversibly detect p-nitrotoluene by oxidative luminescence quenching. For both the thin and thick Films the photoluminescence (PL) is quenched by p-nitrotoluene by similar to 90% in 4 s, which is much faster than that reported for luminescent polymer films. Combined PL and neutron reflectometry measurements on pristine and analyte-saturated films gave important insight into the analyte adsorption process. It was found that during the adsorption process the films swelled, being oil average 4% thicker for both the thin and thick dendrimer Films. At the same time the PL was completely quenched. On removal of the analyte the Films returned to their original thickness and scattering length density, and the PL was restored, showing that the sensing process was fully reversible.