We demonstrate that two large categories of compounds, namely, polyoxometal
lates (produced by acid condensation of mainly tungstates and/or molybdates
) and aggregates of various metal oxides (formed by stirring, sonication et
c) have similar properties. Emphasis is given to the photocatalytic propert
ies of the two systems. The various steps in the photocatalytic cycles are
described and several examples, from both systems, have been used to demons
trate that their photocatalytic performance is similar in terms of the over
all mechanism of photodecomposition of organic compounds, the intermediate
species involved and the final photodegradation products (i.e., CO2, H2O an
d inorganic anions).