The primary purpose of this study is to measure the size- and composition-dependent responses of aerosol surface instruments designed to measure surface area related properties. Measurements were conducted in the range of 30-150 nm of mobility equivalent diameter, Dp. The responses off a LQ1-DC (a diffusion charger manufactured by Matter Engineering AG) and an EAD (a diffusion charger manufactured by TSI) to singlets (NaCl) particles are proportional to Dp1.36 and Dp1.13, respectively. The response of LQ1-DC agrees with Fuchs surface area, which is proportional to Dp1.39 within 2.4% error. The response of the EAD is almost proportional to diameter, Dp. A PAS2000CE (Photoelectric Aerosol Sensor manufactured by EcoChem) gave both size and composition-dependent responses. For diesel particles produced at high engine loads, the response was nearly proportional to Fuchs surface area. However, at lighter engine loads, the response dropped sharply with decreasing D p. Light engine loads are associated with high fractions of volatile particles that may suppress the photoemission response. The secondary purpose of this study is to investigate the difference in charging rate between singlets (NaCl particles) and agglomerates (diesel particles) by using diffusion chargers. Agglomerates (diesel particles at engine load 75%) acquire more charge than singlets (NaCl particles) by 15 and 17% for LQ1-DC and EAD, respectively.