We show that flux pinning in Bi2(Sr,Ca)3−xCu2O8+&dgr;is increased by intentional deviations in cation nonstoichiometry. Polycrystalline samples of alkaline earth‐deficient composition (x=0.24–0.32) show a larger irreversibility in magnetization (&Dgr;M), and a slower decay of &Dgr;Mwith both increasing field and time than stoichiometric compositions. The difference between compositions grows with increasing field out to 5 T, and is greatest atT≤20 K but still significant at 40 K. The critical state model yields a difference in intragranular current density (Jcm) at 20 K and 2 T of ≳102between stoichiometric andx=0.3 compositions. Flux pinning energies determined from relaxation of magnetization are several times greater for cation‐deficient than for stoichiometric compositions. These improvements are only achieved upon slow cooling or low‐temperature annealing, indicating that defect clustering or incipient precipitation is necessary to form more effective pinning defects.