The mixture proportions of pretraining data domains (e.g., Wikipedia, books, web text)  greatly affect language model (LM)  performance. In this paper, we propose Domain Reweighting with Minimax Optimization (DoReMi) , which first trains a small proxy model using group distributionally robust optimization (Group DRO)  over domains to produce domain weights (mixture proportions)  without knowledge of downstream tasks. We then resample a dataset with these domain weights and train a larger, full-sized model. In our experiments, we use DoReMi on a 280M-parameter proxy model to set the domain weights for training an 8B-parameter model (30x larger)  more efficiently. On The Pile, DoReMi improves perplexity acrossalldomains, even when it downweights a domain. DoReMi improves average few-shot downstream accuracy by 6.5% points over a baseline model trained using The Pile’s default domain weights and reaches the baseline accuracy with 2.6x fewer training steps. On the GLaM dataset, DoReMi, which has no knowledge of downstream tasks, even matches the performance of using domain weights tuned on downstream tasks.