A continuous ink jet printer aqueous ink composition including pigment particles dispersed with a dispersant or self dispersing pigment particles without the need for a dispersant, and a polymer additive distinct from any dispersant used to disperse the pigment particles, wherein the polymer additive comprises a styrene-acrylic copolymer that does not substantially displace any dispersant used to disperse the pigment, present in an amount effective to stabilize the ink composition against shear induced agglomeration caused by pumping the ink composition through a continuous ink jet printing fluid system. A method of continuous ink jet printing includes A) providing a main fluid supply of a continuous inkjet printer with an aqueous ink composition of the invention; B) pumping the ink composition from the main fluid supply to a print head and ejecting a continuous stream of the ink composition from the print head which continuous stream is broken into spaced droplets; and C) in response to electrical signals received from a control mechanism, controlling the spaced droplets to select between printing droplets for marking a substrate and nonprinting droplets that are collected and returned to the main fluid supply.