The ground state dipole moment of free base chlorin (H(2)Ch) was determined by dielectric constant measurements of H(2)Ch/benzene solutions (mu(g) = 2.93 +/- 0.08 D). This was combined with the previously measured change in dipole moment (Delta mu(ge) = - 0.255 +/- 0.005 D) associated with the S-1 <-- S-0 (pi pi*) transition to calculate the excited-state dipole moment (mu(e) = 2 68 +/- 0.08 D) of H(2)Ch. These values were used with solvatochromic shift data to find the cube of the cavity radius, a(3), for the H(2)Ch molecule; the assumption was made that the cis and trans tautomers of free base isobacteriochlorin (H(2)iBC) have this same a(3). This number ((4.49 +/- 0.05) x 10(-23) cm(3)) was combined with the spectral shifts and measured Delta mu(ge) vectors to determine the ground and excited state dipole moments of cis-H(2)iBC (mu(g) = 0.35 +/- 0.01 D; mu(e), = -0.97 +/- 0.01 D) and trans-H(2)iBC (mu(g) = 2.16 +/- 0.05 D, mu(e) = 3.51 +/- 0.05 D). Our solvatochromic shift data also indicated that when inferring the sign of a Stark effect determined change in dipole moment, only solvents with low polarizabilities (nonaromatic) should be used.