Abstract We show that the transition to the low temperature ( T ≲ 12 K) magnetic phase (LTMP) of La 2 − x Sr x CuO 4 + y (LSCO) for 0.02 ≲ x ≲ 0.03 is clearly observed via bulk magnetometry in vacuum-annealed samples. The transition temperature T f depends on the strontium concentration and is insensitive to the vacuum treatment. Measurements of an electron spin resonance (ESR) signal are also described. The temperature dependent amplitude of this signal displays a clear anomaly at T f, suggesting that the observed spins participate in the LTMP. Arguments are presented for the intrinsic nature of the observed ESR signal and the LTMP. We compare temperature dependent effects in both probes to expectations for spin glasses, and relate our results to consideration of the nature of the LTMP, and of the origin of the spin system from which the LTMP is formed.