The ultraviolet spectroscopy of isoelectronic pair para-diisocyanobenzene (pDIB) and para-isocyanobenzonitrile (pIBN) has been studied under gas-phase, jet,cooled conditions. These molecules complete a sequence of mono and disubstituted nitrile/isonitrile benzene derivatives, enabling a comparison of the electronic effects of such substitution. Utilizing laser-induced fluorescence (LIF) and resonant two-photon ionization (R2PI) spectroscopy, the S-0-S-1 electronic origins of pDIB and pIBN have been identified at 35 566 and 38 443 cm(-1), respectively. In pDIB, the S-0-S-1 origin is very weak, with b(3g) fundamentals induced by vibronic coupling to the S-2 state dominating the spectrum at 501 cm(-1) (v(17), isocyano bend) and 650 cm(-1) (v(16), ring distortion). The spectrum extends over 5000 cm(-1), remaining sharp and relatively uncongested over much of this range. Dispersed fluorescence (DFL) spectra confirm the dominating role played by vibronic coupling and identify Franck-Condon active ring modes built off the vibronically-induced bands. In pDIB, the S-2 state has been tentatively observed at about 6100 cm(-1) above the So Si origin. In pIBN, the S-0-S-1 origin is considerably stronger, but vibronic coupling still plays an important role, involving fundamentals of b(2) symmetry. The bending mode of the nitrile group dominates the vibronically-induced activity. Calculations carried out at the TD-DFT B3LYP/6-31+G(d) level of theory account for the extremely weak S-0-S-1 oscillator strength of pDIB and the larger intensity of the S-0-S-1 origins of pIBN and pDCB (para-dicyanobenzene) as nitrile groups are substituted for isonitrile groups. In pDIB, a nearly perfect cancellation of transition dipoles occurs due to two one-electron transitions that contribute nearly equally to the S-0-S-1 transition. The spectra of both molecules show no clear evidence of charge-transfer interactions that play such an important role in some cyanobenzene derivatives.