The temperature dependence of transient and persistent spectral holes is reported for the R-lines (E-2 <-- (4)A(2) transitions) in 4% chromium(III) doped NaMgAl(oxalate)(3).9H(2)O. The temperature dependence is well described by taking into account two pseudolocal vibrations in a weak electron-phonon coupling model and the direct process between the two E-2 levels. The persistent spectral hole-burning is based on a nonphotochemical mechanism involving mainly a rearrangement: of a water molecule of crystallization. The 1.12 GHz width of the transient hole in the R-2 line provides a value for the relaxation rate within the E-2 multiplet, k(R-2 --> R-1) = 3.5 x 10(9) s(-1) at 2.5 K. The observed optical line width of Gamma = 46 MHz for the R-1-line at 2.5 K is most likely dominated by chromium(III)-chromium(III) spin-spin interactions. Spectral diffusion on the ms time scale causes persistent spectral holes to be significantly broader and holewidths of 260 MHz and 1.63 GHz for the R-1 and R-2-line, respectively, are observed. Spontaneous hole-filling is thermally activated and spectral diffusion on the minute time scale is slow in the investigated temperature range of 2.5-30 K.