Low temperature magnetization (M) and ac susceptibility (χac) data were used to investigate the reentrant spin-glass state in lengths of melt-spun ribbons of Fe100-xZrx (x=9, 10) amorphous alloys. The temperature range investigated was from 2 to 300K, while the applied magnetic field H was varied in the range of ±85 kOe. The magnetic properties were found to be strongly influenced by the sample composition despite the fact that the amount of Fe varies by 1 at.%. For instance, the Curie temperature (TC) is reduced from 232.5K to 213.0K with decreasing Fe concentration, while M for the lower Fe concentration, measured at the highest applied magnetic field (H=85 kOe) was nearly double the value for that of the higher; the coercivity in the ferromagnetic regime is reduced by a factor close to five when x is increased from 9 at.% to 10 at.%. The ac susceptibility measured for frequencies f in the range of 10-104Hz showed a sharp drop in the magnitude of the in-phase contribution (χ a c ′) and a peak at the out-of-phase component (χ a c ″), which shifts to higher temperatures with increasing values of f. The Voguel-Fulcher law applied to the χ a c ″ data yielded an activation energy Ea/kB=21.3K (40.1K), the glassy temperature TG=15.5K (38.2K), and a relaxation time τ 0 = 9.1 × 10 - 7 s (8.3 × 10 - 7 s), for the sample with x=10 (x=9). A plot of χ a c ″ vs χ a c ′ for a broad range of T and f yielded a broad maximum near the glassy temperature TG for both sample concentrations.