Nucleation in the marine boundary layer affects the distribution of natural sulfur across the aerosol size spectrum and thus may contribute to marine cloud condensation nuclei (CCN) concentrations and climate forcing. We investigated how nucleation probability in remote marine areas develops under different conditions. Our study suggests that conditions favorable for particle formation correspond to transitions from moderate to high relative humidities. The highest nucleation probabilities were associated with transitions that (1) are relatively rapid, (2) occur near the local solar noon, and (3) are a result of the cooling of the air. The probability of these nucleation events occurring is greater under cleaner conditions and is probably more important than those associated with precipitation scavenging in remote marine air. ¿ American Geophysical Union 1994