Recent studies have shown that Kir2 channels display differential sensitivity to intracellular polyamines, and have raised a number of questions about several properties of inward rectification important to understanding their physiological roles. In this study, we have carried out a detailed characterization of steady state and kinetic properties of block of Kir2.1-3 channels by spermine. High-resolution recordings from outside-out patches showed that in all Kir2 channels current-voltage relationships display a 'crossover' effect upon change in extracellular K⁺. Experiments at different concentrations of spermine allowed for the characterization of two distinct shallow components of rectification, with the voltages for half block negative (V¹_1/2) and positive (V²_1/2) to the voltage of half block for the major steep component of rectification (V⁰_1/2). While V¹_1/2 and V²_1/2 voltages differ significantly between Kir2 channels, they were coupled to each other according to the equation V¹_1/2-V²_1/2=constant, strongly suggesting that similar structures may underlie both components. In Kir2.3 channels, the V²_1/2 was ~50 mV positive to V⁰_1/2, leading to a pattern of outward currents distinct from that of Kir2.1 and Kir2.2 channels. The effective valence of spermine block (Z₀) was the highest in Kir2.2 channels while the valences in Kir2.1 and Kir2.3 channels were not significantly different. The voltage dependence of spermine unblock was similar in all Kir2 channels, but the rates of unblock were ~7 fold and ~16 fold slower in Kir2.3 channels than that in Kir2.1 and Kir2.2 when measured at high and physiological extracellular K⁺, respectively. In all Kir2 channels, the instantaneous phase of activation was present. The instantaneous phase was difficult to resolve at high extracellular K⁺ but it became evident and accounted for nearly 30-50% of the total current when recorded at physiological extracellular K⁺. In conclusion, the data are consistent with the universal mechanism of rectification in Kir2 channels, but also point to significant, and physiologically important, quantitative differences between Kir2 isoforms.