Although the company H descriptive roll (10) has James, not Isaac, I am assuming they are the same person, since they otherwise match closely, and since the Pennsylvania Archives Civil War cards don't mention a 'James Tyson' serving in the 91st.
He was 38 years old when he was enlisted (8 [38], 10 [42]). He was born in Philadelphia, Pennsylvania (8, 10).
He was a farmer (8).
He was 5 feet 11 inches tall, and had a dark complexion, light eyes, and dark hair (8, 10).
He was enlisted and mustered into service on 22 or 23 September 1861 (1, 8, 9, 10 [22 Sep]). He weas enlisted for three years, at Philadelphia, Pennsylvania (8, 10). He was mustered in as a private in company D, at Philadelphia, Pennsylvania (8, 10). He had rifle number 16 (10).
He was transferred to company H on 31 October 1861 (7, 8, 9).
He was promoted to teamster at some point (8, 9).
On 3 April 1863, he was reported absent without leave (6).
On 8 September 1863, Sinex reported that he was in Philadelphia and able to rejoin the regiment, and requested his return (2).
He reenlisted as a veteran volunteer on 26 December 1863 (1, 8). He was reenlisted at Bealton, Virginia (8).
On 24 March 1864, Sinex requested that Tyson and Joseph Clowney replace Harry Angel and Charles Armstrong, apparently on a detail (3). They were detailed as teamsters on 26 March 1864 (4).
On 21 September 1864, he was detailed as a teamster and ordered to report to the Brigade AQM (5).
He deserted while on furlough, in February 1865, at Petersburg, Virginia (1, 8).
1 Bates, Samuel Penniman. History of Pennsylvania volunteers, 1861-5. Harrisburg: B. Singerly, state printer, 1869-71. 5 volumes. 'Ninety-first regiment', volume 3, pages 186-233. (In the roster.)
2 letter, Sinex to Marvin, 8 September 1863
3 letter, Sinex to Fowler, 24 March 1864
4 special order 25, HQ 91st PA, 26 March 1864
5 special order 80, HQ 91st PA, 21 September 1864
6 consolidated morning report, 91st PA, 3 April 1863 (Isaac Tyson)
7 company D, register of men transferred, entry 16 (Isaac Tyson)
8 Civil War Veterans' Card File, available at the Pennsylvania State Archives, searched 10 August 2004 (Isaac R Tyson)
9 company H, register of men transferred (Isaac Tyson)
10 company H, descriptive roll, #13 (James R Tyson)
[I did not find an entry for him in the index on <www.ancestry.com> to the pension index by name (searched April 2007)]