The structural effects of incorporating Cs into the monoclinic and tetragonal hollandites Ba1.2-x Cs (x) Mg1.2-x/2Ti6.8+x/2O16 and Ba1.2-x Cs (x) Al2.4-x Ti5.6+x O-16 have been studied using powder neutron diffraction and Cs-133 and Al-27 MAS NMR. Addition of Cs to the monoclinic structure induces a 'shear-type collapse', in agreement with previously published results. NMR spectra show that the addition of Cs does not change the local structure around the Al cations within the tunnel walls. An algorithm is given that allows a prediction of unit cell parameters to be made for tetragonal hollandites containing barium.