The two hallmarks of Alzheimer’s disease (AD) are A-amyloid- (A) plaques and neurofibrillary tangles marked by phosphorylated tau. There is compelling evidence that aggregating A drives tau accumulation, a process that involves synaptic degeneration leading to cognitive impairment. Conversely there is a growing realization that non-fibrillar (oligomeric) forms of Aβ mediate toxicity in AD. Using quantitative proteomics, we find that AD brain sarkosyl-insoluble pellets are greatly enriched with A at almost equimolar levels to N-terminal truncated microtubule binding region (MTBR) isoforms of tau with multiple post-translational modifications (PTMs). The extracted R3-R4 tau peptides are 100-fold higher compared to N-terminus intact tau peptides. Substantial proportions of site-specific phosphorylation at T181 (~ 22 %) and T217 (~ 16 %) along with other PTMs in the proline rich region (PRR) and MTBR indicate a regional susceptibility of PTMs in aggregated tau. Immuno-electron microscopy reveals that co-purified A and tau co-localize to globular non-filamentous aggregates.