A full‐wave analysis of the dispersion in coplanar‐type transmission lines is made including the effect of material dispersion arising from a frequency‐dependent complex dielectric constant such as due to optical phonon resonances in the substrate. The results are used to examine the distortion of femtosecond electrical pulses propagating in coplanar striplines made on ultrathin as well as thick GaAs substrates. A new closed‐form dispersion formula for coplanar lines is also reported, valid for complex dielectric constants and all values of frequency and line dimensions.