We describe a new class of methods for measuring the properties of particles suspended in fluids. We have named these methods modulated dynamic light scattering (MDLS) methods. We present a detailed description of the apparatus and the theoretical foundations for a few examples of MDLS methods in which measured. signals are compared to predictions calculated using Brownian motion theory to infer one or more of the particle mass, diffusion coefficient, velocity, shape category, and other properties. The direct measurement of particle mass is novel. MDLS methods can thus be used in characterizing particles with respect to both mass and surface area, size and shape, or other combinations of properties. MDLS measurements of many individual particles provide the distributions of particles over one, two, or more property values, singly or jointly. To be widely used in particle measurements, the method must measure several thousand in a few minutes. We therefore investigate precision and measurement speed and discuss size range of particle characterization by MDLS. We derive an expression for the precision of an MDLS measurement. Provision of both a measured mass or size of a single particle and an estimate of the measurement precision is also novel. The capability to measure mass, size, and other properties of individual particles at known precision should make MDLS useful in characterizing particles of unknown shape including thin flake-shaped and elongated rod-shaped particles.