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Absolute values for the generalized multiphoton-ionization cross sections and saturation intensities of rare gases are reported for 500-fs, 248.6-nm pulses. For He, Ar, and Kr these values are experimentally determined from the intensity dependence of the ponderomotive shift for intensities comparable to and above the saturation intensity. The measured experimental cross sections [σ(5)He=(0.9+9-o.4)×10-152 cm10 s4, σ(4)Ar=(1.3+1-0.5)×10-116 cm8 s3, σ(5)Kr=(1.2+1.7-0.6)×10-82 cm6 s2, σ(3)Xe=(50-45+350)×10-82 cm6 s2] are found to be in good agreement with theoretical values [σ(5)He=4.4×10-152 cm10 s4 (ab initio), σ(4)Ar=1.1×10-115 cm8 s3 (scaling), σ(3)Kr=0.7×10-82 cm6 s2 (scaling), σ(3)Xe=5×10-82 cm6 s2 multichannel quantum defect theory]. For Ne it is found that a single-rate description is not valid, and therefore the present model for the analysis of the experimental results cannot be applied. The reported Xe experimental cross sections are based on the conventional method of measuring the ion yield vs laser intensity with its concomitant reduced accuracy.