This study presents electrostatically self-assembled nanoparticles from linear flexible polyelectrolytes (poly(diallyldimethylammonium chloride or quarternized poly(4-vinylpyridine)) and an ionic photo-isomerizable azo dye (Acid Yellow 38) that can change their size upon UV-light irradiation. Assemblies with narrow size distribution are stable in aqueous solution. For samples with under-stoichiometric dye load, UV-light exposure triggers a size decrease, e.g. from a hydrodynamic radius of Rh = 94 nm to Rh = 62 nm for an Ay38-PDADMAC sample with a charge ratio of lcharge = 0.7. Size changes are caused by trans-cis isomerization of the dye, accompanied by a change in hydrophilicity, binding enthalpy and entropy. Assemblies are characterized by static and dynamic light scattering, atomic force microscopy, UVvis spectroscopy and isothermal titration calorimetry. Zeta potential measurements give insight into the electrostatic stabilization and size-control of the ionic nano-assemblies, revealing a master curve of effective surface charge density versus hydrodynamic radius. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys., 2013, 51, 802-816