We report on 45 Sc NMR experiments in the geometrically frustrated spinels FeSc 2 S 4 and MnSc 2 S 4 . The temperature dependences of line shift K͑T͒, linewidth ⌬͑T͒, spin-spin relaxation rate 1 / T 2 ͑T͒, and spin-lattice relaxation rate 1 / T 1 ͑T͒ are deduced. The manganese compound reveals the behavior of a long-range ordered antiferromagnet, but with the onset of critical spin fluctuations at unusually high temperatures of 40 T N . K͑T͒ of FeSc 2 S 4 exhibits a cusplike maximum, a hallmark of spin frustration. Orbital fluctuations strongly enhance 1 / T 1 ͑T͒ at elevated temperatures. Toward lowest temperatures these fluctuations slow down resulting in an increase of 1 / T 2 ͑T͒. The exponential decrease of 1 / T 1 ͑T͒ results from the opening of a spin gap ⌬ = 0.2 meV, a characteristic feature of a spin liquid.