We determine full matrices of piezo-optic and elasto-optic coefficients for lead germanate crystals,
P
b
5
G
e
3
O
11
, using Dixon–Cohen and interferometric methods. Two different types of samples are studied, single- and multi-domain. We show that the correct data for the
P
b
5
G
e
3
O
11
crystals can be obtained only with single-domain samples, since the domain structure induces a number of features that make the final results unreliable. We demonstrate that the acousto-optic figures of merit reach high-enough values at the isotropic acousto-optic interactions with longitudinal acoustic waves propagating along the principal
x
and
z
axes (
63
×
10
−
15
and
37
×
10
−
15
s
3
/
k
g
), respectively. At the same time, the efficiency of the acousto-optic interactions with transverse acoustic waves is low. The same is true concerning anisotropic acousto-optic diffraction, since the appropriate elasto-optic coefficients are small enough.