The performance of Partially Premixed Compression Ignition (PPCI) combustion relies heavily on the proper mixing between the injected fuel and the in-cylinder gas mixture. In fact, the mixture distribution has direct control over the engine-out emissions as well as the rate of heat release during combustion. The current study focuses on investigating the pre-combustion equivalence ratio distribution in a light-duty diesel engine operating at a low-load (3 bar IMEP), highly-dilute (10% O₂), slightly boosted (P_in =1.5 bar) PPCI condition. A tracer-based planar laser-induced fluorescence (PLIF) technique was used to acquire two-dimensional equivalence ratio measurements in an optically-accessible diesel engine that has a production-like combustion chamber geometry including a reentrant piston bowl. Equivalence ratio distributions are presented at a single vertical plane and three different horizontal planes within the combustion chamber, including two planes within the bowl of the piston. The quantitative measurements provide a detailed picture of the mixture formation process for an early injection PPCI combustion regime. Based on the measured equivalence ratio distributions, the mass of engine-out UHC and CO were approximated and the results confirm that the main source of UHC and CO emissions is from over-mixed, overly-lean fuel/air mixtures. The measured equivalence ratio distributions were also combined with previous in-cylinder velocity and CO measurements to describe the mechanism by which these overly-lean mixtures are formed and how they are transported to the squish region late in the engine cycle.