In one of the first studies of isolated intermediates in protein aggregation, we have used circular dichroism and fluorescence spectroscopy to characterize metastable oligomers that are formed in the early steps of β-lactoglobulin heat aggregation. The intermediates show typical molten globule characteristics (secondary structure content similar to the native and less tight packing of the side chains), in agreement with the belief that partly folded states play a key role in protein aggregation. The far-UV CD signal bears strong resemblance to that of a known folding intermediate. Cryo-transmission electron microscopy of the aggregates reveals spherical particles with a diameter of about 50 nm and an internal threadlike structure. Isolated oligomers as well as larger aggregates bind the dye thioflavin T, usually a signature of the amyloid superstructures found in many protein aggregates. This result suggests that the structural motif recognized by thioflavin T can be formed in small oligomers.