Fxperimental I-V curves of microwave-driven Josephson tunnel junctions with resistive shunts are reported. The results are in very good agreement with numerical calculations using the resistively shunted junction model. In the low-frequency regime there are three distinct regions in the I-V curves and two different types of Shapiro steps. It is demonstrated that the overall shape of the I-V curves can be explained by using an adiabatic interpretation of the junction response. The two different types of Shapiro steps are related to a compensation effect between dc and rf bias currents.When a Josephson junction is driven by a microwave field, two main features are experimentally observed: first a progressive changing of the overall shape of the I-V characteristic with the microwave power, and at the same time, the appearance of' Shapiro steps' corresponding to the locking of the Josephson oscillator to the oscillating external field. Numerous experiments and theoretical calculations have been devoted to the magnitudes of the low-order Shapiro steps. A Bessel-function dependence of the steps on the microwave power is usually expected from a simple voltage source model. However, experimentally, Josephson junctions are often current-biased and deviations from the Bessel-function behavior have been reported and also predicted by using the resistively shunted Josephson-junction model (RSJ) without capacitance. ' However, not much has been done in the past concerning the overall shape of the I Vcharacteristic-s of microwave-driven junctions and, to our knowledge, a physical interpretation of the non-Bessel-function behavior of the Shapiro steps and of their position on the I-V curves has not been previously reported. In this communication, we present detailed experimental results on resistively shunted Josephson junctions driven by microwave radiation in the low-frequency regime (co &co" where co, is the characteristic frequency given by co, =2eAI, /A). The shape of the I Vcharacteristics aswell as the step behavior are not only in very good agreement with numerical simulations using the RSJ model, but are also explained in a physical way. The 0.15 & 10-pm Nb-NbOX-PbInAu edge junctions shunted by a TiAu strip used in this experiment were fabricated by Brown et al. using their standard processing. " The parasitic inductance of the resistive shunt was deliberately reduced by using a superconducting ground plane. Typical values of the junction parameters were I, =0.15 mA, R=2Q, and C=0.4 pF, where I"R,and C are, respectively, the critical current, the resistance, and the capacitance of the junction. Due to the low value of the Stewart-McCumber parameter P, =2eR CI, /R=-O 7, . no capacitance-induced hysteresis was observed in the I-V characteristic.For p, &1, the junctions can be closely simulated by using the RSJ model without capacitance.The junctions were irradiated with microwaves at frequencies around 12, 18, and 36 GHz. Figure 1 shows the I-V characteristics for different microwave powers at v=18 GHz. When the microwav...