Water activity-temperature state diagrams for Lactobacillus acidophilus freeze-dried in a sucrose or a lactose matrix were established based on determination of stabilized glass transition temperatures by differential scanning calorimetry during equilibration with respect to water activity at fixed temperatures. The bacteria in the lactose matrix had higher stabilized glass transition temperatures for all a(w) investigated. The survival of Lactobacillus acidophilus determined as colony forming units for up to 10 weeks of storage at 20 degrees C for (i) a(w) = 0.11 with both freeze-dried matt-ices in the glass), state, (ii) a(w) = 0.23 with the bacteria in the lactose matrix in a glassy state but with the bacteria in sucrose matrix in the nonglassy state, and (iii) a(w) = 0.43 with both freeze-dried matrices in a nonglassy state showed that the nature of the sugar was more important for storage stability that? the physical state of the matrix with the nonreducing sucrose providing better stability than the reducing lactose. (C) 2009 American Institute of Chemical Engineers Biotechnol. Prog., 25: 265-270, 2009