[en] We present a scheme of deterministic mediated superdense coding of entangled photon states employing only linear-optics elements. Ideally, we are able to deterministically transfer four messages by manipulating just one of the photons. Two degrees of freedom, polarization and spatial, are used. A new kind of source of heralded down-converted photon pairs conditioned on detection of another pair with an efficiency of 92% is proposed. Realistic probabilistic experimental verification of the scheme with such a source of preselected pairs is feasible with today's technology. We obtain the channel capacity of 1.78 bits for a full-fledged implementation. - Highlights: • Deterministic linear optics mediated superdense coding is proposed. • Two degrees of freedom, polarization and spatial, are used. • Heralded source of conditioned entangled photon pairs, 92% efficient, is proposed.