The incident occurred at around 7.30pm on October 11 at Bond Street Underground station, when a woman is reported to have entered the station with her sister and two young children.
A man and woman were walking ahead of them before the woman collided with the buggy which contained one of the children.
According to the British Transport Police (BTP), the man began to shout racial abuse at the family before the woman then allegedly grabbed the victim by her hair.
The man is reported to have produced a bottle and said it was pepper spray as the victim tried to defend herself.
The unknown substance in the bottle was then sprayed in the direction of the family, who reported no ill effects following this.
Before both parties left, the woman is said to have shouted abuse and made lewd gestures.
Officers believe the pair pictured could have information that might assist enquiries.
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Enquiries to establish what the substance was are ongoing.
Anyone who recognises them is asked to text BTP on 61016, using the reference 413 of 12 October.
Information can be given anonymously to Crimestoppers on 0800 555 111.
