We present Gran-Telescopio-Canarias/OSIRIS optical spectra of 4 of the most compact and massive early-type galaxies in the Groth Strip Survey at redshift z \sim 1 , with effective radii { R _ { \mathrm { e } } } = 0.5 - 2.4 kpc and photometric stellar masses M _ { \star } = 1.2 - 4 \times 10 ^ { 11 } M _ { \odot } .
We find these galaxies have velocity dispersions \sigma = 156 - 236 \mathrm { km s } ^ { -1 } .
The spectra are well fitted by single stellar population models with approximately 1 Gyr of age and solar metallicity .
We find that : i ) the dynamical masses of these galaxies are systematically smaller by a factor of \sim 6 than the published stellar masses using BRIJK photometry ; ii ) when estimating stellar masses as 0.7 \times M _ { \mathrm { dyn } } , a combination of passive luminosity fading with mass/size growth due to minor mergers can plausibly evolve our objects to match the properties of the local population of early-type galaxies .