The minimum activation volumes for magnetooptic (MO) films of different thicknesses have been determined. If nucleation of a magnetic reversal is assumed to be cylindrical, then the minimum diameter that nucleates the reversal appears to be in inverse proportion to the film thicknesses. This may indicate that the maximum storage density achievable with these materials may be dependent on their thickness. Analysis of the field variation of the activation volume indicates that the thinner film may have a more uniform grain volume. Comparison between activation volumes obtained and those obtained for similar materials with different compensation point temperatures suggest that activation volumes are quite a strong function of temperature/composition. This is important in materials as, by the nature of the recording process, magnetic reversal takes place at elevated temperatures. It is noted that activation volumes are potentially useful as a method of determining the minimum domain size. In addition, the form of the fluctuation field can give information on the distribution of anisotropy and grain volumes.