Dehydration of porous glass hydrated with a 5 mM solution of the spin-label molecules 2,2,6,6-tetramethyl-4-acetamido-piperidine-1-oxyl (TMAPO) was studied with electron paramagnetic resonance at 9 GHz. The spin-label molecules' concentration remains constant, i.e., it follows the Langmuir isotherm when glass pores of 292 +/- 20 nm diameter are dehydrated. When the pore diameter is only 127 +/- 11 nm, or less, the Langmuir isotherm appears not to apply despite the fact that in smaller pores the surface-to-volume ratio is larger. The reason for this apparent failure could be the establishment of a water cluster phase which does not dissolve TMAPO, and/or it could be a kinetic effect of the spin-label molecules. In this report the possibility that the failure of the Langmuir isotherm is the consequence of ordered water clusters is examined. To account for the lower concentration of TMAPO in smallest pores, as much as 70% of total water volume has to consist of such water clusters, in agreement with an earlier proposal. (C) 1997 American Institute of Physics. [S0021-9606(97)52647-X].