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We report the development and validation of a new humidified aerosol single-scattering albedometer to quantify the effects of water uptake on submicrometer particle optical properties. The instrument simultaneously measures in situ aerosol light extinction (σep) and scattering (σsp) using a cavity-attenuated phase shift-single scattering albedo particulate matter (PM) monitor (CAPS-PMSSA, Aerodyne Research, Inc., Billerica, MA, USA). It retrieves by difference aerosol light absorption (σap) and directly quantifies aerosol single-scattering albedo (SSA), the aerosol "brightness." We custom built a relative humidity (RH) control system using a water vapor-permeable membrane humidifier and coupled it to the CAPS-PMSSA to enable humidified aerosol observations. Our humidified instrument (H-CAPS-PMSSA) overcomes problems with noise caused by mirror purge-flow humidification, heating, and characterizing cell RH. Careful angular truncation corrections in scattering, particularly for larger particles, were combined with empirical observations. Results show that the optimal operational size to be Dp < 400 nm. The H-CAPS-PMSSA was evaluated with several pure single-component aerosols including ammonium sulfate ((NH4)2SO4), absorbing nigrosin, and levoglucosan, an organic biomass smoke tracer. The measured σep, σsp, and the derived optical hygroscopicity parameter (κ) for size-selected ammonium sulfate are in good agreement with literature values. For dry size-selected nigrosin in the 100 < Dp < 400 nm range, SSA values increased from ∼0.3 to 0.65 with increasing Dp. The enhancement in nigrosin σap at RH = 80% was a factor of 1.05–1.20 relative to dry conditions, with the larger particles showing greater enhancement. SSA increased with RH with the largest fractional enhancement measured for the smallest particles. For polydisperse levoglucosan, we measured an optical κ of 0.26 for both light extinction and scattering and negligible absorption. Our new instrument enables reliable observations of the effects of ambient humidity on mixed aerosol optical properties, particularly for light-absorbing aerosols whose climate forcing is uncertain due to measurement gaps. Copyright © 2021 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]