We had the following case: Crystallization conditions: 1.56M AS + 100mM PIPES, pH 5.7 Cryo conditions: 40% (w/v) PEG-400 + 100mM PIPES, pH 5.7 Xal survives in the latter solution for 10 minutes, not more. We simply soaked for 7 to 10 minutes and freezed. Works well. Tuomo Glumoff Centre for Biotechnology, Turku, Finland