We investigate the electron and hole spin relaxation in an ensemble of self-assembled InAs/In0.53 Al0.24 Ga0.23 As/InP quantum dots with emission wavelengths around 1.5 μm by using pump-probe Faraday rotation spectroscopy. Electron-spin dephasing due to the randomly oriented nuclear Overhauser fields is observed. At low temperatures we find a submicrosecond longitudinal electron-spin relaxation time T1 which depends unexpectedly strongly on temperature. At high temperatures the electron-spin relaxation time is limited by optical phonon scattering through spin-orbit interaction decreasing down to 0.1 ns at 260 K. We show that the hole spin relaxation is activated much more effectively by a temperature increase compared with the electrons.