The transient radiation-induced attenuation (RIA) of two different versions of pure-silica-core (PSC) multimode optical fibers (so-called “solarization-resistant” fibers) exposed to nanosecond 1 MeV X-ray pulses are investigated. On-line RIA spectra measurements at both room temperature (RT) and liquid nitrogen temperatures (LNT) in the range 1–3.5 eV are performed. Following the RIA kinetics, the properties of the metastable defects that are bleached just after the pulse are discussed. The spectral decomposition of the RIA is performed using known Gaussian bands associated to point defects absorbing in this spectral range. For both fiber types, the generation and the post-irradiation kinetics of the self-trapped holes (STHs) that are the main contributor to the transient RIA are investigated particularly. Moreover, thanks to the measurements performed at low temperatures the complex temperature dependence of the kinetics of self-trapped holes and some chlorine-related species is characterized.