Investigations of charge carriers trapping in semiconducting oxides such as titanium dioxides and similar systems is a topic of high interest for various areas of scientific research that include electrochemistry, heterogeneous catalysis and photocatalysis. Electron magnetic resonance (EMR or EPR) techniques are suitable tools to investigate both electron trapping and electron hole trapping in oxides since, in most cases, these phenomena originate paramagnetic states. In the present contribution some selected examples will be illustrated concerning: a) chemically induced excess electrons, b) stabilisation of photogenerated electrons, c) stabilisation of photogenerated electron holes. The delicate balance between charge localisation (small polarons) and delocalized free-carriers states will be briefly discussed in the limits permitted by the magnetic resonance technique.