Conducting composites of polyaniline doped with dodecylbenzenesulfonic acid (PAni-DBSA) and poly(styrene-b-styrene-butadiene-b-styrene) (STF) as supporting matrix were prepared by in situ synthesis. The influence of composition and thickness on the electromagnetic properties and shielding effectiveness (SE) of the materials was evaluated using a microwave network analyzer and a standard rectangular waveguide from 8.2–12.4 GHz (X–band). It was found that the composition 49/51 exhibited the best SE and the higher values of reflected power (PR), ε′, ε″ and ε*. The compositions 12/88 and 17/83 presented the higher values of transmitted power (PT) and absorbed power (PA), respectively. The composition 25/75 presented the higher value of tan δ. The thickness of 5× showed the best SE and higher value of PR. The higher value of transmitted power (PT) was obtained for thickness of 1×. The values of PA were similar for all thicknesses. The thickness of 1x showed the higher values of ε′, ε″, ε* and tan δ.