This paper presents a range of experimental data acquired from inside the combustion chamber of a high-pressure, steady-flow confined spray burner. Corresponding measurements are compared with results from a spray-combustion model. The data corresponds to samples taken from locations at different distances from the injector tip with an axial fuel spray and co-flowing air. Values of soot concentration (or soot volume fraction) and dimensions of primary particles in soot agglomerate, are given. Diesel fuel was used in the tests, which were conducted at pressures between 1.1 and 2.1 MPa, with overall input mixture equivalence ratios ranging from 0.8 to 1.2. Primary-particle size distributions and mean particle diameters, in the agglomerate samples, were determined for the different locations, using transmitting electron microscopy and associated image-analysis diagnostics. A computer-simulated oxidation process was applied to measured size distributions from upstream locations, to compare with actual measured distributions obtained at downstream locations. Agreement was found to depend on local conditions. Evidence of internal oxidative attack is demonstrated for some individual primary particles within the agglomerates, under certain operating conditions, through a widening of the inter-crystallite spacing.