Germicidal compositions containing phenylmalonaldehyde-type compounds, or mixtures of phenylmalonaldehyde-type compounds and phthalaldehydes, and methods of using such compositions for killing bacteria, disinfection, or sterilization, are disclosed. In one aspect, a germicidal composition may include a diluent, and a germicidal compound having the formula: ##STR00001## wherein Ar is an aryl group that is selected from the group consisting of phenyl, 4-pyrimidinyl, and 2-(2-nitro-3-formyl-phenyl). In a further aspect, the composition may also include a germicidal efficacy enhancer such as isophthalaldehyde or a combination of isophthalaldehyde and terephthalaldehyde.