This contribution presents the synthesis and properties of four thiophene-containing poly(-p-arylene-ethynylene)/poly(-p-arylene-vinylene)s, PIa-b and PIIa-b, whose repeating units (RU) consist either of 1:2 or 2:2 triple bond / double bond ratio, and which bear linear alkoxy side chains not longer than octyloxy and branched 2-ethylhexyloxy. PIa-b and PIIa-b exhibit similar absorption and emission behaviour in dilute solution (lambda(a) = 483-486 nm,lambda(e) = 540 nm) as well as in solid state (lambda(a) = 500, 530 nm,lambda(e) = 560 nm), whereby slightly higher fluorescence quantum yields (Phi(f)) were obtained for PI than for PII systems, as a result of higher number of thiophene units within the RU of PII An enhancement of the Phi(f)-value from 0% to 3% is obtained after replacing linear octadecyloxy in PIIc-e by bulky branched 2-ethylhexyloxy in PIIa-b. Nonoptimized solar cells of configuration ITO/PEDOT:PSS/polymer: PCBM (1:3 weight ratio)/LiF/Al show open circuit voltages as high as 900 mV for PIa-b and 800 mV for PIIa-b. Reducing the size of the side chain from R = 2-ethylhexyl in PIa to R = methyl in PIb leads to a significant increase of the short circuit current, I-sc, from ca. 2.5 mA to ca. 3.7 mA and consequently to an enhancement of the energy conversion efficiency, eta(AM1.5), from ca. 1.2% to ca. 1.7%. This is due to an extended donor-acceptor interfacial area, as evidenced by AFM topology pictures showing smaller nanoscale clusters size in PIb than in PIa active layer. The same change led to minimal effect in PII systems. (c) 2007 Wiley Periodicals, Inc.