N,N′-disubstituted-4,4-bipyridinium (viologen) compounds and polyoxometalates (POMs) are two large groups of photochromic materials. In this work, we studied the photochromic property and colour modulation property of hydrogen bonding POMs-viologen hybrids. Taking the reported (H 2 pipz) 3 -[Mo 8 O 27 ] (1: no viologen containing) and (C 14 H 11 N 4 O) 2 [Mo 8 O 26 ](2) as important examples and combining with the two reported POM-viologen crystalline hybrids, we have demon- [a]