A model based on Mie theory is described that estimates bulk participate refractive index n ¯ p from in situ optical measurements alone. Bulk refractive index is described in terms of the backscattering ratio and the hyperbolic slope of the particle size distribution (PSD). The PSD slope ξ is estimated from the hyperbolic slope of the particulate attenuation spectrum γ according to the relationship γ ≈ ξ − 3, verified with Mie theory. Thus the required in situ measurements are the particulate backscattering coefficient, the total particulate scattering coefficient, and the particulate attenuation coefficient. These parameters can be measured with commercially available instrumentation with rapid sampling rates and real-time data return. Application of the model to data from the Gulf of California yielded results that agreed with expectations, e.g., predicted n ¯ p was 1.04–1.05 in the chlorophyll maximum and 1.14–1.18 near sediments. Below the chlorophyll maximum in case I type waters, predicted n ¯ p values were between 1.10 and 1.12, suggesting the presence of a significant inorganic mineral component in the background or detrital organic particles with low water content.