Hypothesis: Since the volume fraction of the surfactant bilayer(s), of thickness d(b), in a vesicle and liposome is smaller than one, the dependences of the Rayleigh (R) scattering intensity and turbidity on the particle radius a are weaker than those for a homogeneous sphere, which are proportional to a(3). The dependences of the Rayleigh-Debye-Gans (RDG) scattering intensity and turbidity on a are also weaker. Work done: The dependences of the effective relative refractive index on a, d(b), and d(w) (water layer thickness) were derived. The specific Rayleigh ratio R-theta** and the specific turbidity tau** for single and independent scattering were derived analytically for R and RDG scattering. Spectroturbidimetry data at 25 degrees C for a cationic double-chain surfactant, didodecyldimethylammonium bromide (DDAB) were compared to the turbidity predictions. Findings: For R scattering, R-theta** and tau** are proportional to a(2)d(b) for vesicles, and to a(3) (d(b/)d(w)+d(b)) for liposomes. For RDG and particle radii 20-1000 nm, tau** is proportional to a(n), where n is 2 to 0.4 for vesicles and 2 to 1.1 for liposomes. Turbidity data for DDAB vesicles are consistent with the RDG predictions, which are also used to estimate the vesicles' sizes. RDG applies to liposomes < 800 nm and to much larger sizes for vesicles. (C) 2020 Elsevier Inc. All rights reserved.