Comparisons between the Engine Exhaust Particle Sizer Spectrometer (EEPS) and scanning mobility particle sizers (SMPS) have shown that the EEPS underestimates size and concentration of particles > similar to 75 nm and that the discrepancies vary by particle type. This paper describes the development of a new EEPS instrument matrix to improve size distribution measurements for compact-shape particles. Both the operating principle and the mathematical model of the EEPS data inversion are described. Monodisperse and polydisperse sucrose, poly-alpha-olefin oil (PAO), and sodium chloride (NaCl) particles were used for calibration. A new instrument matrix was developed and implemented for data inversion. This study confirms the literature findings that the geometric mean diameter, geometric standard deviation, and total number concentration by EEPS with the default instrument matrix (referred to as IM-2004) agrees with SMPS within +/- 20% for compact-shape particles < similar to 75 nm. However, EEPS with IM-2004 underestimates geometric mean diameters by 20-40% for larger particles. The revised instrument matrix improves the size agreement with the SMPS to within +/- 14% across the entire size range. (C) 2015 Elsevier Ltd. All rights reserved.