The nonlinear optical properties of self-assembled monolayers obtained from bonding two different alpha-functionalizedterthiophenes (alpha-T-3)to (111) silver electrode surfaces have been investigated using second harmonic generation (SHG). The two (alpha-T-3) compounds used were functionalized with alkane chains of different lengths (C-8 and C-4), and each was terminated with a thiol anchoring group. A nitrile group was attached to the terminal thiophene ring of the (alpha-T-3) compound with the C4 chain. The orientation of the polarization of the incident beam was changed systematically and gradually between "p" and "s" orientation and the SH signal (isotropic and anisotropic contributions) analyzed in both directions ("P" and "S"). The symmetry of the system was reduced by the presence of the adlayers from C-3v to C-3. The dependence on the applied potential and the incident wavelength has also been studied. The relative magnitudes and phases of the various second-order tensor elements have been estimated and compared with the values for a bare surface. A resonance process in the (alpha-T-3) pi moiety has been investigated, and from this, the effective "band-gap" energies of the organic semiconductor SAMs (i.e., the energy difference between the pi-pi* bands) have been estimated.