Aqueous colloidal poly(N-isopropylacrylamide-co-4-vinylpyridine) [poly(NIPAM-co-4-VP)] copolymer microgels have been synthesized using different percentages of 4-vinylpyridine (4-VP). A surfactant-free emulsion polymerization reaction using N-isopropylacrylamide (NIPAM) and 4-vinylpyridine comonomers cross-linked with N,N'-methylenebisacrylamide was utilized. The reaction was initiated using the cationic initiator 2,2'-azobis(2-amidinopropane) dihydrochloride. Transmission electron micrograph data show the copolymer microgels to be monodisperse spheres. The pH and electrolyte sensitivity of the copolymer microgels have been studied, as well as temperature sensitivity, since microgels undergo a reversible volume phase transition in response to heating and cooling. Changes in the hydrodynamic diameters in the system were monitored as a function of temperature (25-60 degreesC), pH (3-8), and ionic strength (10(-3)-10(-1) mol dm(-3) NaCl or NaClO4) using photon correlation spectroscopy. The hydrodynamic diameter of poly(NIPAM-co-4-VP) microgels increases with decreasing pH, as the vinylpyridine units become more protonated. However, the hydrodynamic diameter decreases with increasing ionic strength (over the pH range 3-8) and with increasing temperature (at pH 3 and pH 6). UV-visible spectrophotometry measurements showed a good correlation between the molar absorption and the percentage of vinylpyridine incorporated. Potentiometric titrations were used to determine the pK(a) values of the copolymer microgels.