The inhibition performance of different phosphonates as CaCO 3 scale inhibitors in simulated cooling water was evaluated. Based on experimental data, various phosphonates form weakly soluble Ca–phosphonate precipitates, with the order of the phosphonates in terms of Ca 2+ tolerance being BHMTPMP>PAPEMP>HDTMP≫PBTCA>DTPMP>EDTMP>ATMP>HEDP. Homo-, co-, and ter-polymers were also investigated for their performance as Ca–phosphonate inhibitors, the order of inhibit Ca–phosphonate precipitation: terpolymer>copolymer>homopolymer. Addition of polymers as inhibitors with phosphonates could reduce Ca–phosphonate precipitation and enhance the inhibition efficiency for CaCO 3 scale. Moreover, polymer composition and concentration are critical to the inhibitory performance.