Solid acids such as A1H(2)P(3)O(10)(.)2H(2)O, Zr(HPO4)(2)(H2O)-H-., arnorphous-AIPO(4), and A1H(3)(PO4)(2)(.)3H(2)O showed low conductivities of 10(-8)- 10(-7) S cm(-1) at 130 degrees C, whereas mechanical milling for these solid acids was found to improve the conductivities by 2 to 4 orders of magnitude. On the other hand, a large amount of phosphorus was leached out during a durability test after milling. XRD showed the solid acids were transformed to amorphous or crystal-amorphous composites by mechanical milling. P-31 MAS NMR spectra indicated that the intensities of the bands due to Q(3) and Q 4 units decreased and that of the peak due to Q(0) units increased. The increase in conductivity and the decrease of chemical durability of the mechanically milled solid acids are probably caused by the fort-nation Q0 units, i.e., orthophosphoric acid, in the resultant composites due to the disruption of inorganic structures. (c) 2005 Elsevier B.V. All rights reserved.