This invention provides novel β-lactamase inhibitors of the aryl- and heteroaryl-sulfonamidomethylphosphonate monoester class having nitrogen-based cations or quarternary ammomium groups. The compounds inhibit three classes of β-lactamases and synergize the antibacterial effects of β-lactam antibiotics (e.g., imipenem and ceftazimdime) against those micro-organisms normally resistant to the β-lactam antibiotics as a result of the presence of the β-lactamases.