The present study evaluated the influence of different radiopacifiers on an established mineral trioxide aggregate (MTA)-like Portland cement system. Commercial MTA contains bismuth oxide (Bi2O3) as the radiopacifier and inert alternatives tested were barium sulphate (BaSO4), lanthanum oxide (La2O3) and tantalum pentoxide (Ta2O5). The radiopacity of the different formulations was measured with densitometry, whilst relative porosity was determined using helium pycnometry and compressive strength was measured as an indicator for the potential longevity of the alternative cement formulations. At 20wt% radiopacifier content the order of radiopacity was, Bi2O3 (3.71mm Aluminium equivalent), Ta2O5 (2.76mm Al), La2O3 (1.85mm Al) and BaSO4 (1.48mm Al). The commercial MTA control had a radiopacity of 3.65mm Al. The addition of all radiopacifiers caused a deterioration in strength and increased porosity; the incorporation of 20wt% Bi2O3 reduced strength from 68MPa by 36%. Ta2O5-containing cement had a strength comparable with that containing Bi2O3 (43MPa), whilst La2O3 and BaSO4 addition generated even weaker cements. Commercial MTA had a similar strut density to the Bi2O3- containing model system although had a lower strength (34MPa) due to its higher porosity. Bi2O3 appeared to be the best of the investigated radiopacifiers for this endodontic filling material as it provided the highest mechanical stability, lowest porosity and was the most radiopaque.