As many antibiotics are ionizable and may have different dissociation forms in the aquatic environment, we hypothesized that the different dissociation species have disparate photolytic pathways, products, and kinetics, and adopted ciprofloxacin (CIP) as a case to test this hypothesis. Simulated sunlight experiments and matrix calculations were performed to differentiate the photolytic reactivity for each dissociation species (H4CIP(3+), H3CIP(2+), H2CIP(+), HCIP(0), and CIP(-)). The results prove that the five dissociation species do have dissimilar photolytic kinetics and products. H4CIP(3+) mainly undergoes stepwise cleavage of the piperazine ring, while H2CIP(+) mainly undergoes defluorination. For H3CIP(2+), HCIP(0), and CIP(-), the major photolytic pathway is oxidation. By density functional theory calculation, we clarified the defluorination mechanisms for the five dissociation species at the excited triplet states: All the five species can defluorinate by reaction with hydroxide ions (OH(-)) to form hydroxylated products, and H2CIP(+) can also undergo C-F bond cleavage to produce F(-) and a carbon-centered radical. This study is a first attempt to differentiate the photolytic products and mechanisms for different dissociation species of ionizable compounds. The results imply that for accurate ecological risk assessment of ionizable emerging pollutants, it is necessary to investigate the environmental photochemical behavior of all dissociation species.