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Three-step resonance ionization mass spectroscopy has been performed to observe the even-parity Rydberg series converging to four states of Gd + (4f 7 5d6s( 10 D o 5/2 ), 4f 7 5d6s( 10 D o 7/2 ), 4f 7 5d6s( 10 D o 9/2 ) and 4f 7 5d6s( 8 S o 7/2 )). Using low-angular-momentum states as the second-step intermediate levels, simple and unperturbed series structures have been obtained. From an analysis of these structures, the first ionization potential of atomic Gd has been determined with improved accuracy as 49 601.45 ± 0.35 cm −1 . In addition, on the basis of the isotope shifts of the Rydberg states the isotope shift ( 160 Gd-154 Gd) of the ionization potential of −77 ± 20 mK has been found. †