The size-resolved composition of nanoparticles formed and grown through acid-base reactive uptake has been studied in the laboratory by reacting gas-phase nitric acid (HNO3) and dimethylamine (DMA) in a flow tube under dry (<5% RH) and humid (∼55% RH) conditions. Size-resolved nanoparticle composition was measured by a thermal desorption chemical ionization mass spectrometer over the diameter range of 9-30 nm. The nanoparticle geometric mean diameter grew in the presence of water compared to dry conditions. Acid/base ratios of HNO3-DMA particles at all measured sizes did not strongly deviate from neutral (1:1) in either RH condition, which contrasts with prior laboratory studies of nanoparticles made from sulfuric acid (H2SO4) and base. Theoretical methods were used to investigate the underlying chemical processes that explain observed differences in the compositions of HNO3-DMA and H2SO4-DMA particles. Calculations of HNO3-DMA cluster stability indicated that a 1:1 acid/base ratio has >107 smaller evaporation rates than any other acid/base ratio in this system, and measured nanoparticle composition confirm this to be the most stable pathway for growth up to 30 nm particles. This study demonstrates that nanoparticle formation and growth via acid-base reactive uptake of HNO3 and DMA follow the thermodynamic theory, likely because of both components' volatility.