Using interacting boson model, shape phase transitions from U(5) to O(6) and from U(5) to SU(3) are studied in the space of two control parameters. Some simple shape phase diagrams in terms of the order parameter and one of the control parameters are presented. Spectra and E2 transition rates are used to construct observables. It is shown that these observables exhibit the behavior of the shape phase transition, namely a peak structure at and after the critical point.