In October 1936 he went to Spain (arriving on 20 October) as a member of the first group of volunteers to fly in the Spanish Civil War.
In Spain, he served in Escuadrilla Turzhanskii, which was equipped with I-15s.

By 9 December, he was credited with 2 and 1 shared He 51s.

On 2 February 1937, the five remaining I-15s of the detachment from the Escuadrilla Rychagov (I-15) at Malaga flew out to explore the roads in the Roda-Burgo area. During the flight, they noticed three enemy fighters, identified as He 51s, flying from Seville to their airfield of Antequera (30km north of Malaga). They chased after them, catching them over the airfield. One of the Nationalist pilots turned towards the Republican fighters but all three were quickly shot down. The Nationalist pilot turning on them was shot down as a shared between Leitenant Yemel’yan Kondrat and Matyunin. Leitenant Nikolay Artemyev claimed the second in flames while the third was claimed by Yevgeniy Yerlykin, who reported that the combat started at 6,500 meters. Yerlykin’s I-15 was damaged in the combat.
The fifth pilot in the patrol was a Spanish pilot, who attacked one of the Heinkels head-on but his I-15 was seriously damaged with one of the cylinders in the engine destroyed. He managed, however, to return to base.

Matyunin left Spain on 27 March 1937.
During his time in Spain, he claimed 2 and 4 shared victories flying the I-15.

Claims:

Kill no.	Date	Time	Number	Type	Result	Plane type	Serial no.	Locality	Unit
	1936
1	??/11/36		1	He 51	Destroyed	I-15		Madrid	Escuadrilla Turzhanskii
2	??/11/36		1	He 51	Destroyed	I-15		Madrid	Escuadrilla Turzhanskii
	??/11/36		1/?	He 51	Shared destroyed	I-15		Madrid	Escuadrilla Turzhanskii
	1937
	02/02/37		1/2	He 51	Shared destroyed	I-15		Antequera	Escuadrilla Rychagov
	05/02/37		1/4	He 59	Shared destroyed	I-15		Malaga	Escuadrilla Rychagov
	??/??/37		1/?	CR.32	Shared destroyed	I-15		Spain	Escuadrilla Rychagov

Biplane victories: 2 and 4 shared destroyed.
TOTAL: 2 and 4 shared destroyed.

Sources:
Air Aces Home Page - Jan Safarik
Polikarpov I-15, I-16 and I-153 Aces - Mikhail Maslov, 2010 Osprey Publishing, Oxford, ISBN 978-1-84603-981-2
Soviet airmen in the Spanish civil war 1936-1939 - Paul Whelan, 2014 Schiffer Publishing Ltd, ISBN 978-0-7643-0
Spanish Republican Aces – Rafael A. Permuy López, 2012 Osprey Publishing, Oxford, ISBN 978-1-84908-668-4