Temperature dependences of linear electro-optic coefficients in two isomorphous ferroelectric crystals (CH3NH3)Bi2Cl11 (PMACB) and (CH3NH3)5Bi2Br11 (PMABB) have been measured around temperatures of their ferroelectric-paraelectric phase transitions. The employed external reference measuring method (Mach-Zehnder interferometer) allowed for determination of values of all main free Pockels coefficients at room temperature.